Links for Math 5 - 3386
Here's a virtual manipulative for experimenting with triangles to learn about congruence. See why the congruence tests (SSS, SAS, ASS, AAS) actually work. It shows that if you have, for instance, 3 fixed-length sides (SSS), there is a unique triangle you can construct with these. http://nlvm.usu.edu/en/nav/frames_asid_165_g_1_t_3.html?open=instructions
Wolfram has some basic geometry demonstrations.
Whistler's Alley has some geometric constructions.