There are so many of variants out there, but a Duemilanove or Diecimila is a good starting point, as are a few Freeduinos with the same form factor, USB, and voltage regulators.
I was wondering about variants last week so put together a chart of 32 available Arduino-compatible boards on my little Arduino blog-- Jeff's Arduino Blog: Comprehensive Arduino-Compatible Hardware List?
It grew to 41 in a more detailed spreadsheet for comparing criteria, including a few boards that are questionably "Arduino-compatible" in terms of having the bootloader and/or not making all "regular" pins available for general use-- that's the "maybe not?" column. 
Good luck and happy hacking!
-Jeff