If you went to school at all in the twentieth century, chances are you had a set of wooden shape blocks to practice pattern making or tessellations with.  The set of yellow hexagons, orange triangles, green squares, and tan diamonds was well loved in my experience but could cause serious problems when a two gallon box got spilled all over the floor.  The National Council of Teachers of Mathematics has solved that problem with their interactive set of shape blocks.  Just like the original, these blocks can be rotated, flipped, and interlocked.  There is also an option to select a pattern to recreate with no chance of spilling hundreds of tiny blocks onto the carpet.

Website: http://illuminations.nctm.org/Activity.aspx?id=3577
Subject: Math
Grade Level: K-5
Price: Free