Arduino Pro Mini
Finding the traditional USB Arduino a bit too big? This board includes everything the larger board does (except the USB to Serial portion) and does so in 1/6th the space (18 x 33mm). With the reduced size and cost it makes slipping an Arduino into almost anything possible. It also comes with the headers unpopulated letting you either solder in headers (for use in a breadboard) or your own wires directly to the board.
Available from oomlout: