TI Launchpad / MSP 430 Development
TI Launchpad is a very cheap (€3,85 per board incl. mini-USB cable!) alternative to Arduino based on TI MSP430 MCU.
Note that Launchpads ship with an external crystal that you need solder to the board yourself (if you plan on using the boards for anything involving a clock).
There are addons for the Launchpad that plug into the pins available on the Launchpad (just like Arduino Shields). Currently available:
Energia - Arduino IDE clone for MSP430
osx-launchpad - MSP toolchain for OS X