The AVR series of microprocessors has quickly become my CPU of choice. The family has a wide selection of parts, from 8 pin DIPs to large flat packs.
I'm using the Atmel STK500 development kit
(PDF). It was only $79 from Digikey
The STK500 comes with an excellent assembler/debugger IDE, and supports a wide range of AVR devices and programming modes. Development is quick and easy.
I've also used the AVR Butterfly, which is a very inexpensive ($20!), but feature
packed AVR device. It can be programmed with WinAVR
excellent port of the GNU GCC compiler. The
Butterfly is also available from Digikey.
I used the Butterfly to make a solar powered weather station that collected data without a hitch for three of the hottest months of the year in the Mojave desert.