Many moons ago I've worked with 8-bitters like 68HC05 and Z80, later AVR and MSP430 (16-bit). However most recent projects were on ARM7. Several manufacturers offer ARM controllers, in all colors and sizes (well, not really color).
ARM(7) is replacing 8-bit architecture: it's more performant (32-bit RISC at faster instruction cycles than most 8-bitters), has more memory and is available with several IO-configurations.
I worked with NXP LPC2000 controllers, which are also inexpensive (< 1 USD for a 32-bitter!).
If you're in Europe http://www.olimex.com/dev/index.html has some nice low-cost development boards. Works in the rest of the world too :-)