Pattern blocks can be used to teach patterns and algebra, symmetry, fraction concepts and other math topics.
http://nlvm.usu.edu/en/nav/frames_asid_169_g_1_t_2.html?open=activities&...

See
http://math.rice.edu/~lanius/Patterns/notes.html
and
http://mason.gmu.edu/~mmankus/talks/PBlocks/pbact/other.htm
for some ideas on how to use this manipulative in the classroom. This is a nice Jave implementation of this manipulative.