Metrix Create:Space has several different microcontrollers available for purchase or tool rental.
We generally have Arduinos in our vending machine. We usually have a couple of different form factors for the Arduino. We'll always have the UNO version, and sometimes we'll have some more minimalist versions of the Arduino platform.
To learn more about the Arduino, check out our series of workshops.
We have two Launchpad development boards for the MSP430. These boards are available as tool rentals. If you're interested in learning how to use them, let us know. If we get enough interest, we'll put together a workshop about them.
http://processors.wiki.ti.com/index.php/MSP430_LaunchPad_Mac_OS_X and http://mspdebug.sourceforge.net/faq.html#rf2500_osx