Non-manufacturer microcontroller selection site?
Digi-Key has a broad search tool where you can select parts across 30 microcontroller manufacturers. It has ARMs, AVRs, PICs, 8051 derivatives etc. -- over 48,500 individual parts.

(Click for a larger view)


You can search for core size (8, 16, 32), speed, peripherals, program size, RAM size, EEPROM size, # of I/O pins, and package across all parts.