What math related gift can I give an elementary school teacher?

Any books by Bonnie Tank or Marilyn Burns, if they don't already have them.

I also think that "problem of the day" or "daily puzzler" books are great fun and good inspiration.

I've seen tesselation puzzle pieces that are made from famous escher drawings--could be a cool gift and fun for the classroom. I saw them sold in a museum store but I've heard you can also buy them online.

the game "rush hour" for the classroom, or one of those giant transparent calculators that goes on an overhead projector.

A really nice abacus.

oh, also the book "the number devil" -- it's a german storybook about math.

Also The Math Curse, by John Scieszka.

"Flatland" by Edwin (or Edward? can't remember) Abbott Abbott (yes, his middle name is the same as his last name) is an excellent novel that manages to tie in philosophy, sociology, faith, religion and gender issues with a painless introduction to dimensional geometry on a level accessible to teenagers on up. A great read, and potential source for a wide variety of teaching projects in several content areas.

"Flatland", as hossman said above, is a really good read. What you might also want to check out is a psuedo-sequal, "Flatterland". Both have wikipedia articles:
http://en.wikipedia.org/wiki/Flatland, http://en.wikipedia.org/wiki/Flatterland

One other GREAT math-gift book, one that I myself actually gave to an old math teacher of mine as a gift, is "The Joy of Pi" - has it's own website:
http://www.joyofpi.com/