D-Link used to sell one - DWL-122. It is now discontinued but some suppliers may have stock. It is only a 802.11b product. The drivers on the CD which come with it are not the latest ones, which you can download from their site. However, the drivers/product are not compatible with Tiger or later versions of 10.3.x
Incubus, I've been using the D-link DWL-122 with a B&W G3 running 10.3.8(?9) and also a Mac mini running 10.4.2. No problems (other than the icon in the system preferences looking really bad), provided that the latest driver is used. Also 10.4 and 10.4.1 developed(!) kernel panics after a while.
You can try what I just did. I used a belkin wireless bridge to hook up to my existing wireless network. You have to configure it with the SSID and encryption of your network, and also the wireless MAC address of the router you want to hook it to so it knows where to connect (because it's a bridge, not a client). Then connect any ethernet PC to it and it should just work without any configuration of the PC. They are about $40. It may only work with G routers, and possibly only belkin G routers, I don't know.