I have a good experience with Microchip PIC24 and dsPIC family. It's easy to learn, GNU based C compiler, cheap ICD 2 programmer/hardware debugger, free TCP/IP stack.
Try PIC24, if you need DSP feature try dsPIC. There are Microchip 16 Bit Design contest now, sponsored by C.Cellar n Microchip. For who got the free sample kit, they can buy the ICD2+28 pin starter board for just $99, normal price is about $240.
just skip the 16 bit stuff and go to a 32 bit arm chip. futurlec sells a nice little arm7 board for under $25. Their pic and atmega and 8051 boards cost more. And you can downlaod a complete gnu toolset for it.