For those doing realtime networked games...

[Corun]

http://www.valve-erc.com/srcsdk/general/multiplayer_networking.html

It's probably been posted before. But, it's quite good anyway :-).

[OneSadCookie]

Also well worthwhile, the Quake 3 networking model,

http://trac.bookofhook.com/bookofhook/trac.cgi/wiki/Quake3Networking

and an article about how the original UT does it:

http://unreal.epicgames.com/Network.htm