Teensy USB Development Board
Serial ports are, sadly, a bit out of date. That's a shame because it was pretty easy to work with serial ports -- especially since the advent of chips like the MAX232 that didn't require negative voltage supplies.
Of course, it is easy enough to get a USB to serial cable and you are back in business. But that looks a bit Rube Goldbergish. There are also some nice chips from FTDI that are basically the guts of a USB to serial cable so you can make a more polished product.
However, I saw an inexpensive development board that looks like it would be fun although I haven't tried it myself. The tiny board (costs less than twenty bucks) has a 16MHz AVR chip onboard and has tools for Windows, Linux and Mac. I may just have to try one.