I would also recommend AVR (8-bit) and Butterfly or DB101 kit. The main advantage is that there is a GCC compiler available and that you can program them through the Serial Port, without the need of a tool. Inexpensive programming and debugging tools are also available. There is a very strong AVR community in AVRFreaks
Another alternative is ARM7 and ARM9 microcontrollers (32bit). If you are interested in using an OS (ucLinux/FreeRTOS for ARM7, Linux for ARM9), you should go that way. There is of course a free GCC compiler. You can buy kits and tools at Olimex
