Online

Aso Brain Games
Offer Xplorers Settlers, a java based version of Catan. Also offer Toulouse, Brainiac, and Roadblock. [Java]
http://games.asobrain.com/

Eurosettlers The Xplorers Clan.
Gaming group who play on the Asobrain Xplorers (java-based) system. Includes rules, member roster and a guestbook.
http://www.freewebs.com/eurosettlers/

jsettlers.com: Java Settlers of Catan
Play ladder games, or random games versus humans and/or AI robots. Uses the JSettlers software, hosted on two servers at jsettlers.com.
http://www.jsettlers.com/

NetSet: Network Settlers
Play Settlers over the net. Program only runs on Windows systems.
http://home.earthlink.net/~mgardner0/index.html

Settlers Project Website
Play over the net. Requires Linux or Solaris.
http://settlers.seul.org/

SourceForge.net: JSettlers
Open source software for Java Settlers of Catan. Forum, bug tracking, screen shots, and downloads.
http://sourceforge.net/projects/jsettlers/

Yahoo Groups: Settlers of Catan
Play by email for basic game and Cities and Knights expansion. Registration fee, requires download.
http://groups.yahoo.com/group/settlers-of-catan/