Factorial Lookup. Ever wonder what 182,731! is? (You might have to give it several seconds to download before it renders.) Sometimes the most practical way to solve difficult (or impossible) problems is to just try to look up the answers in a massive precalculated table. Also see Reverse MD5 Hash Lookup.
Arimaa—a modern game played on a chess board with chess pieces, specifically designed to make it difficult to write effective AI to play it.
Making Computer Science More Enticing. A good inspirational 2-minute vid at the top, and information about how Stanford University's revamping of their CS program led to higher enrollment.
OGRE 3D is a good bit of software, it's not an all out game engine, but it's one heck of a rendering engine. The only problem is dealing with the CMake based builds (hopefully improving), and porting to platforms that lack dependencies, that the build system assumes as a must have, even though they are actually optional. FreeBSD builds of trunk in particular suck.
I wouldn't want to support multiple OSes with it, but under Windows at least, it works quite well.