When setting up my new office recently one of the test machines wound up being a distance away from the rest of the systems and the wired network. Rather than going through the hassle of dropping a CAT-6 line to this test station, the quicker and easier approach was to just pickup another USB WiFi adapter. The wireless adapter ended up being the MediaLink USB54G that offers USB 2.0 and 802.11g support, but only mentioned compatibility for Microsoft Windows 2000, XP, and Vista systems. How did the MediaLink USB adapter working out under Linux? Quite well.
If you want to extend an existing wired network, you could also look at Homeplug technology, that uses ordinary power wiring to carry Ethernet data. All Homeplug devices are plug 'n' play, OS-independent, except for if you want to set up AES encryption :-(
After buying a motherboard with an RTL8187 that has reliability issues when uploading, and buying a D-link DWL-G122 stick that has low speeds (also uses r73usb driver), I bought the Homeplug and I'm pretty happy with it. And it's network transparent so it's guaranteed that no kernel updates will break it.