Tune Buster is a free keyboard synthesizer for OS X, and can turn your musical USB keyboard into a complete instrument. Tune Buster is easy to use and no configuration is needed, just start the program, plug in a keyboard and play.
Different sounds can be selected by clicking the desired instrument in the popup menu. Use the arrow keys or the mouse scroll button if you have one to speed up the selection process.
Tune Buster can be made invisible by closing the main window, and the keyboard will still play. Highly recommended - simple yet effective. Th easiest way to play a keyboard through your Mac that I have ever seen. I like Garageband et al, but this fills a gap - lightweight, easy to use, very handy to have if you just want to play and don't want to launch any of the big boys. I actually just let it sit in the dock, and my keyboard is always ready to play.
