This 802.11g WiFi adapter, which is backed with a 5-year warranty by Encore, had "just worked" with Ubuntu 8.10. The Realtek 8187B ASIC was detected and the rtl8187 Linux driver was loaded on the Linux 2.6.27 kernel. The mac80211-based version of this Realtek driver has been in the Linux kernel since 2.6.23.
The Encore ENUWI-G2 doesn't have an external antenna or any other unique features, but for $15 USD it's a reputable 802.11g USB WiFi adapter. We've tested it with Ubuntu 8.10 without problems thus far.