WavIRCX is an open source IRCX server with a nice interface. For advanced users, you can also use notepad and edit the config file manually! Config files are in an easy to use .ini format.
WinSE- Windows IRC Services. Currently being ported from VB6 to VB.NET/C#. Contains BotServ, ChanServ, NickServ, etc. as well as some interesting features to limit operator abuse.
gClient is a free, open source IRC client for Windows and later Linux.
pureIRCd is an IRC Server written C (though throughout the years i've used various programming languages, including VB6, VB.net C#, and am now stuck with C) to provide a fresh IRC (RFC1459) implementation and avoid the clutter of existing solutions.