Series of Harvard architecture RISC microcontrollers from Microchip. Looking like innocent TTL logic chips, they get tiny-to-huge interface/control jobs done with flair, speed, and low power consumption. They are the CPU for the nifty Basic Stamps--running a powerful token interpreter in under 1024 bytes.

The 16F84 is a very popular part: configured with FLASH RAM and programmable with free software / (very) minimal HW. Once code is stable, OTP chips bring the cost to nothing.

Inexpensive Compilers/emulators/programmers (see GNUpic) abound. If you do Windows, get the excellent MPLAB for all your editing and simulating needs. Free CD with datasheets and software for the asking from Microchip.