Two new Arduino-compatible boards are out and are now listed in my big spreadsheet of boards.
The ArduPilot Mega by SparkFun ($60) is an extension of the original ArduPilot, but as its name implies, now sports an ATmega1280 as its main processor. Designed to be the brains behind a UAV or radio-controlled flying vehicle, it's all ready to hook up to a GPS receiver, servos, RC I/O, accelerometers and gyros, etc. It's amazing how much is packed in there-- it even has a secondary ATmega328 and multiplexer.
From Spikenzie Labs, the folks who make the Voice Shield and Altoids tin-sized Prototino comes a "SIP" (Single In-line Package) Arduino-compatible board "Sippino" ($16 kit). It can be mounted vertically or horizontally in a breadboard and has a 6-pin FTDI breakout header for programming, though no power regulation or reset.