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Cut the Knot
Cut the Knot!: Interactive Mathematics: Geometry Articles, Theorems, Problems
Contains over five hundred Geometry articles, problems, puzzles, and games. Some sites relating to circles are provided within this list.
Other
Juegos, Acertijos, Y Recreaciones Matematicas
In Spanish. A variety of logic problems, including puzzles and word problems with links to the answers. An excellent site to engage Spanish speaking students.