If you need additional USB ports or have a beige G3 or run OS X on an unsupported system using XPostFActo, you can use CompUSA's $15 no-name two-port USB card. On my PowerMac 7600 it worked right out of the box. I was able to print from it to my Epson inkjet printer.
A Web search revealed the card, an MP-861-2, to be made by Megapower International Corp. in Taiwan. Their online information makes no mention of OS X, by the way.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2002020721552984