I've used these motors on several robots and they work out great!
http://www.goldmine-elec-products.com/prodinfo.asp?number=G17222
At 12V a pair of these are perfect for a medium sized robot base. For any of you that saw my trash can robot at the Mini Maker faire or at the regular Maker Faire it used a pair of these in the base.
At only $3.49 each this is a good time to stock up. I'm sure these would work in a lot of other projects too.
Robert