The folks at
Electronic Design Magazine have an interesting article describing small factor microcontroller boards and modules titled
Buy your own Small-Form-Factor Boards and Modules. For quick inexpensive prototypes that are not too complex, I would also consider boards from:
I've only worked with the Kontron product quite a few years ago but the were well made and worked well. Before that, I worked with a uCDimm form factor processor board but the issue with this board is that it did not have a MMU which limits the OS selection.