1-2-3 blocks (aka setup blocks) are a highly configurable drill press and milling fixture. They are most useful when you need a straight edge or a fixed support surface. Normal 123 blocks are made of hardened steel ground perfectly square and flat. These plastic ones are a nice cheap alternative. (Steel blocks on amazon for comparison: http://amzn.to/28PFOgM)

The block dimensions are 1” x 2” x 3”. The holes (5/16") are used to fasten the blocks to each other or to your table. 

Setup blocks go along nicely with step blocks (clamp set): http://www.thingiverse.com/thi...