My good friend Jeff Frohwein has started production of his MultiBoot Version 2 cable, which allows you to download up to 256 kbytes of code to your Gameboy Advance and test your programs on the actual hardware. I have one and I must say it is a wonderful development tool.
Update: The first batch of 50 cables sold out rapidly, but there are more batches on the horizon, check www.devrs.com/gba periodically for more information.