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.
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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.