Elaine over at Elaine Adair Pieces asked me about the kaleidoscope ruler in my comment on her blog. The one that I have is designed by Marti Mitchell and will do up to 16" blocks using the marked lines, but you could fudge and easily do 18" blocks. It will also do little ones as small as 6 inches. The ruler looks like this:
These are easy, fun, and an excellent stash-buster, much better if not so matchy-matchy, in my opinion.