Faldon - Faldon is an online RPG I wrote. The game has some flaws, but I encourage you to try it.
Faldon Client Rewrite - I haven't had time lately to work on it, but I was rewriting Faldon's client some time ago. Details here.
Apache Portable Runtime - APR helps keeps a server portable, and has handy functions for safe path manipulation.
cURL - cURL is a reliable HTTP library.
Percival RSA - The smallest RSA implementation I know of. Floating point, so check your rounding mode.
SDL - SDL abstracts video, input, etc., allowing one to avoid tedious low-level coding.
SQLite - This is a *great* featureful and free embeddable database engine.
Squirrel - Squirrel is a nice object-oriented scripting language.
TinyXml - TinyXml acts a lot like .NET's XmlDocument class. For an XML library it's pretty small (140KB), and the speed is adequate.
XmlRpc++ - XmlRpc++ is a compact, functional XML-RPC library.
XML-RPC.NET - XML-RPC.NET is a complete implementation for the server side.
DeepSea Obfuscator - For C# programs this is the best code obfuscator I have found. The obfuscation rarely breaks code and its developers are responsive with fixes when problems crop up.
Helpinator - Helpinator is an easy-to-use CHM/PDF/HTML documentation builder. It's able to save in a source-control-friendly manner.
NSIS - NSIS is an installer builder from Nullsoft, the creators of Winamp. It supports BZip2 and 7-Zip compression and produces EXEs only a few kilobytes larger than a raw archive.
Bit Twiddling Hacks - Full of clever bit manipulation algorithms.
Chris Hecker's Website - Chris's writings on game design are very insightful. Not updated very often.
lw-plugin - Go here if you have any questions about writing plugins for Lightwave 3D.
Molly Rocket - One of those rare coding forums where the programmers are actually competent.
MUD-Dev Archive - MUD-Dev was a list server full of thoughtful discussions about online RPGs. I have it archived on this site.
Scientific Software and Graphics Algorithms - Algorithms and papers on filtering, graphics, root finding, many handy topics really.
The Inner Product - This site has many interesting ideas and articles. Sadly, it hasn't been updated recently.
Illumination Pit - Zammbi has lots of nice rendering art on his page.
Spectre's Software - Aaron has a lot of good games and utilites here. Have a look.
Yet Another Blog - Crovax is a good friend and a GM on Faldon. This is his weblog.