Click on two cards to turn them over. If they match, they are cleared from the board. If not, they remain on the board. How many turns will it take you to clear the board?

http://www.interactivestuff.org/match/maker.phtml?featured=1&id=15

This is a very cool and very engrossing set of puzzles. Use the cubes to build the shape that has the correct front, top and side views. Now try to solve the puzzle using the minimum number of cubes. The shape can be spun in 3-dimensions. Three thumbs up!

http://www.fi.uu.nl/toepassingen/02015/toepassing_wisweb.en.html

This virtual manipulative allows you to display, rotate, and resize Platonic solids. It also allows you to select vertices, edges, and faces, and observe that the number of vertices minus the number of edges plus the number of faces is always equal to 2 (Euler's formula).
http://nlvm.usu.edu/en/nav/frames_asid_128_g_1_t_3.html?open=instruction...