After trying to find a simple and elegant way to make my applications talk to each other over the network (and being tired of digging through all the socket related C API every time), I found a very nice network abstraction framework.
ThoMoNetworking is completely Objective-C, requires less then ten lines of code to use, and works both on the desktop and on all of Apple's mobile devices (iPhone and iPad). A disk image with the framework libraries, source code, documentation and examples are available from the developers' site. Some of the limitations of the library are also pointed out there, and you should read them before using it.
ThoMoNetworking gives you client and server classes that connect automatically via Bonjour - nothing to do on your side. Pass an object to the server, and it will pop out at the client, and vice versa. If you want, have your object distributed to all connected clients in one go. It couldn't be much simpler. ThoMoNetworking is under an MIT-like license so you can use it in almost any way you want. The full license text is included in the download package.
[crarko adds: I haven't tested this one, but did look through the disk image to make sure the source code was there, for debugging and verification purposes. You may also need to rebuild this if the target SDK changes.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20100526072410319