Dragon Go Server: DGS is a place where you can play turn-based Go games. It functions more or less the same way as playing Go games via email or postcard would be. You play via a simple web interface: no Java, JavaScript or anything to install. Free.
Jörch's Go: Play 9x9 Go online with computer programs Jogo and Wally. Uses pure HTML; no Java required.
Online-Go: Turn-based Go website/server. Includes challenges, mini-tournaments, different rules and board sizes.
World Web Go: Play Go Online.
Yahoo Go: Web-based go server with basic features: ability to start, join and watch games of various sizes and options.