Changing the Border
This is a nifty puzzle whose solution is based on the area of a triangle. The land is divided into two parts (P and Q) by the border ABC. Redraw a straight border line passing through point A so that each of the areas does not change.
http://www.ies.co.jp/math/java/geo/tochia/tochia.html

Dividing a Quadrilateral
This is a more challenging problem that is based on the same underlying concept.
http://www.ies.co.jp/math/java/geo/shi2to/shi2to.html

Practice estimation skills by determining the number of objects, the length of a line, or the area of a shape. Choose the problem type, or choose 'Random' to see a mixture of all three types of estimation questions.

http://www.shodor.org/interactivate/activities/Estimator/

Tangrams are a versatile manipulative. They can be used to build understanding of geometry, fractions, area and other topics. This is a Java implementation of tangrams.
http://nlvm.usu.edu/en/nav/frames_asid_268_g_1_t_3.html?open=activities&...