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, and 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.

2313Temper8 is an AVR assembly language program that reads up to 8 DS1820/DS18S20 devices connected to Port B pin 2 of an AT90S2313 and calculates the extended temperature for the DS18S20 selected. The software uses the '2313's UART to communicate with a host computer at 9600,8,N,1.

AVR2LCD is an AVR assembly language program  that connects an AT90S2313 to a standard alphanumeric LCD that uses a Hitachi (or equivalent) HD44780 display driver. It only uses three port lines, thanks to a 74HC164 8 bit shift register. Code and schematics are included in the zip file.

Atmel AVR Page
AVR-Tutorial  Excellent AVR assembly language projects.
AVR Embedded Microcontroller Resources
AVR Projects by Martin Thomas. Martin has ported the AVR Butterfly application code to avr-gcc. He also has example code.
WinAVR is a port of the GNU GCC compiler. The Windows version comes with Programmer's Notepad, and excellent programming editor.