I have gathered many lists of online resources for geometry. The games and activities are grouped topics—please see the following menu.
This page itself contains a list of general geometry websites, worksheets, and books.
Angles
Coordinate plane
Congruent transformations
Similar figures
Circle / Pi
Constructions
The Pythagorean Theorem
General geometry websites
Worksheets
Books
Advanced topics
General geometry websites
Geometry - Math Warehouse
Detailed lessons about angles, triangles, quadrilaterals, circles, similar triangles, parallelograms, polygons, and trapezoids.
www.mathwarehouse.com/geometry
Geometry course from Learning Math
This online geometry course includes readings, problems, videos, interactive activities, homework problems and solutions. It is meant for K-8 teachers but will work well for middle school students as well.
www.learner.org/courses/learningmath/geometry
Absorb Mathematics
Absorb Mathematics is an excellent interactive course written by Kadie Armstrong, a mathematician. The lessons in it contain interactivity, explanations, and quiz questions. The course concentrates on geometry and trigonometry but includes a few other topics as well. While most of the content is accessible by a fee only, some lessons are available as free samples.
www.absorblearning.com/mathematics/contents.html
BBC Bitesize Shape and space
Short revision (review) "bites" with some activities and short tests for middle/high school level. Topics include shapes, angles, transformations, lines, polygons, symmetry, circles, Pythagorean Theorem, and constructions.
www.bbc.co.uk/schools/ks3bitesize/maths/shape_space
Geometry Homework Help from Math.com
Basic tutorials and quizzes on common geometry topics found in middle and high school.
www.math.com/homeworkhelp/Geometry.html
Geometry Reference Sheet
Both online and printable versions; includes area and volume formulas for common shapes plus Pythagorean Theorem.
www.eeweb.com/toolbox/geometry-reference-sheet
Geometry Tutorials
Simple tutorials on triangles and their properties, polygons, symmetry, angles and much more. Also includes lots of solved geometry questions and some interactive applets.
www.analyzemath.com/geometry.html#tutorials
Dynamic Geometry® Explorations
A bunch of interactive geometry activities, along with instructions and questions to guide your exploration. Need a Java-enabled browser.
math.kendallhunt.com/x19812.html
National Library of Virtual Manipulatives for Interactive Mathematics: Geometry
Collection of interactive geometry activities: Congruent triangles, fractals, geoboard activities, Golden rectangle, Ladybug leaf, Ladybug mazes, platonic solids, tangrams, tessellations, transformations and more.
nlvm.usu.edu/en/nav/category_g_3_t_3.html
Great Pyramid
Clear instructions on how to make a scale model of the GreatPyramid at Giza out of paper.
mathforum.org/alejandre/mathfair/pyramid2.html
Key to Geometry Workbooks
Here is a non-intimidating way to prepare students for formal geometry. Key to Geometry workbooks introduce students to a wide range of geometric discoveries as they do step-by-step constructions. Using only a pencil, compass, and straightedge, students begin by drawing lines, bisecting angles, and reproducing segments. Later they do sophisticated constructions involving over a dozen steps-and are prompted to form their own generalizations. When they finish, students will have been introduced to 134 geometric terms and will be ready to tackle formal proofs.
Worksheets
Math Mammoth Geometry Worksheets Collection
A collection of quality worksheets with variable problems for grades 3-8. Topics include angle relationships, triangles, quadrilaterals, congruency, similar figures, constructions, area, volume, and the Pythagorean Theorem. Price: $9.00 download.
www.mathmammoth.com/worksheets/geometry.php
Geometry worksheets & quizzes
Worksheets about complementary and supplementary angles, parallel, perpendicular, and intersecting lines, types of angles, basic shapes, area & perimeter of rectangles, and parts of a circle.
www.dadsworksheets.com/v1/
Geometry worksheets & quizzes
A bunch of PDF worksheets on geometry topics for elementary level, plus online quizzes.
www.math4children.com/Topics/Geometry
Maisonet Math
Worksheets and online quizzes on a variety of geometry (and other) topics. Look for the links in the left sidebar for Angles, Area, Circumference, Geometry, Transformations, and Volume.
www.mrmaisonet.com
MathKinz - Geometry
A few free geometry worksheets, most are for grades 2-5.
mathkinz.com/Geometry.php
High School Measurement Worksheetsand Geometry Worksheets
From FunMaths.com website, written by teachers from all over the world.
www.funmaths.com/worksheets/math_measurement_06.htm and
www.funmaths.com/worksheets/math_geometry_02.htm
Let's Practice Geometry - Free Worksheets
Lots of free worksheets for high school geometry, including topics such as the Pythagorean Theorem, perimeter, area, volume, angle relationships, triangle theorems, similarity and congruence, logic, proofs, trig, polygons, and circles.
www.letspracticegeometry.com/free-geometry-worksheets
Geometry worksheets at AGMath.com
Hundreds of free PDF worksheets for high school geometry topics, including geometric constructions, triangle congruence, circle, area, the Pythagorean Theorem, solid geometry, and similarity. The worksheets are loosely based on the "Discovering Geometry" textbook by Michael Serra. Compiled by math teacher Jason Batterson.
AGmath.com/1973.html
Key to Geometry Workbooks
Here is a non-intimidating way to prepare students for formal geometry. Key to Geometry workbooks introduce students to a wide range of geometric discoveries as they do step-by-step constructions. Using only a pencil, compass, and straightedge, students begin by drawing lines, bisecting angles, and reproducing segments. Later they do sophisticated constructions involving over a dozen steps-and are prompted to form their own generalizations. When they finish, students will have been introduced to 134 geometric terms and will be ready to tackle formal proofs.
Books
Math Mammoth Geometry 1
A textbook/workbook together for grades 3-5. You will learn geometry by drawing. Topics covered include angles, rectangles, parallelograms, polygons, circles, area, perimeter, and volume. Price: $6.90 download, $12.70 printed.
www.mathmammoth.com/geometry_1.php
Math Mammoth Geometry 2
A textbook/workbook together for grades 6-7. It continues the study of geometry after Math Mammoth Geometry 1, continually emphasizing conceptual understanding, besides calculation-type exercises. Price: $5.70 download, $10.40 printed.
www.mathmammoth.com/geometry_2.php
Geometry: A Guided Inquiry
An excellent choice for high school geometry. I've written an in-depth review of this book and of its supplement Home Study Companion - Geometry, latter by David Chandler.
www.amazon.com/gp/product/0895824302
Harold Jacobs Geometry
A popular choice for a geometry textbook among homeschoolers, and well written. Please read my review.
www.amazon.com/gp/product/0716743612
Dr. Math Introduces Geometry: Learning Geometry is Easy! Just ask Dr. Math!
A supplement to middle school geometry, Dr. Math Introduces Geometry has a light tone, clear layout, and humorous cartoons by Jessica Wolk-Stanley. The best part is that the explanations to math questions are accurate and clear. You'll find answers to dozens of real questions from students who needed help understanding the basic concepts of geometry. This book covers angles, triangles, quadrilaterals, area, perimeter, Pi, circle parts, polyhedra, surface area, volume, nets, congruent transformations, and symmetry. Price ~$12 new, ~$1-6 used.
www.amazon.com
Dr. Math Presents More Geometry
An inexpensive companion to any high school geometry course with excellent explanations. Read my review.
www.amazon.com/gp/product/0471225533
Kiselev's Geometry / Book I. Planimetry
A famous Russian geometry book translated. Acclaimed for the clarity of exposition which makes the book accessible for 7th graders.
www.amazon.com/gp/product/0977985202
A High School First Course in Euclidean Plane Geometry
This is a high school geometry course by Charles Aboughantous. It is based on Euclid's five postulates and his most common theorems. It promotes the art and the skills of developing logical proofs. Practice problems are included at the end of each chapter in three groups: geometric construction problems, computational problems, and theorematical problems. The answers to the computational problems are included at the end of the book. The solutions manual of the book contains detailed solutions to all the problems in the book. It is free to home schooled students upon proof of purchase of the book. Samples are found at https://home.earthlink.net/~draboughantous/geometry.html
www.amazon.com/gp/product/1599428229
Advanced topics
Go Geometry
All things geometry: geometry theorems, huge list of geometry problems, presented with visual diagrams, geometric art, puzzles, dynamic geometry, and more.
gogeometry.com
Why is High School Geometry so Difficult
And what to do about it. In this article, I investigate the reasons and remedies for why so many students find their high school geometry course to be difficult.
/teaching/geometry.php
Tim's Triangular Page
Lists all the basic facts about triangles followed by some problems.
sakharov.net/triangle.html
How to Write a Solution
An article exploring aspects of how to write a clear solution - or proof. Meant for college students but helpful for anyone who wants to learn to write mathematical proofs.
www.artofproblemsolving.com/Resources/
Euclid's Elements
A set of Java applets that bring to life the theorems in Euclid's Elements.
aleph0.clarku.edu/~djoyce/java/elements/elements.html
NonEuclid
Interactive Java software for creating ruler and compass constructionsin both the Poincaré disk and the upper half-plane models of hyperbolic geometry. See also, Why is it Important for Students to Study Hyperbolic Geometry?
cs.unm.edu/~joel/NonEuclid/NonEuclid.html
Online math resources menu
Elementary
Addition and subtraction
Place value
Clock
Money
Measuring
Multiplication
Division
Math facts practice
The four operations
Factoring and number theory
Geometry topics
Middle and high school
Fractions
Decimals
Percent
Integers
Exponents
Probability
Geometry topics
Algebra
Calculus
Trigonometry
Logic and proof
For all levels
Favorite math puzzles
Favorite challenging puzzles
Math in real world
For gifted children
Math history
Math games and fun websites
Interactive math tutorials
Assessment, review, and test prep
Online math curricula