Apple AirPort Express...awesome little device, and it is smaller (1.5 decks of cards) than all of the others because there is no separate power brick, it has retractable AC prongs. Not only does it provide 802.11g, but it provides USB printer sharing and iTunes streaming audio out to an audio system.

http://www.apple.com/airportexpress/

Runs about $129 most places, works with Mac and Windows PC's...