The GGZ Gaming Zone offers a complete free infrastructure for online games. It has always provided some sort of KDE integration, however starting from KDE 4.0, this integration will be available out of the box from the KDE Games Library (libkdegames).
A number of tutorials were sent to the kde-games-devel list.
While KDE 3-based development of GGZ games requires the ggzmod library, KDE 4 has kggzmod and thus the libraries and all games using it will compile without any further dependencies. In order to be able to play the games from an online gaming client (the so-called GGZ core clients such as KGGZ), the game client still needs to be registered with the GGZ game modules registry. The tool ggz-config, which is part of the ggz-client-libs package, is needed for this task. Most distributions ship it as part of a ggzcore-bin package or similar. The file module.dsc contains meta information about the game, such as its author, UI library/environment it runs in and where its executable is located. The two important values are which protocol it uses and which version of the protocol.
If and only if those two match the values of the game server, this game client will be offered to players! More information about this file can be found in the ggz-config readme file and in the GGZ game development guide (which contains a lot more information and should really be printed out). Please consider re-using existing protocols for equal or similar games. The three places to look at are: KDE Games, GNOME Games, and the games shipped by the GGZ project in their various packages. On http://www.ggzgamingzone.org/engines/ there is a list of all known GGZ games.
See the tutorials linked above. There is not that much new information right now. It is however charming to know that KReversi received GGZ support and thus can serve as a codebase where to get ideas from. The second example would be GGZ's KConnectX game which is supposed to be replaced by a GGZ-enabled KWin4.