Mondriaan produces a set of images that look a lot like the paintings of the artist Piet Mondriaan. By way of selection and genetic operators this set can be manipulated by you. You can try to find images that you think look even more like Mondriaan art, or search for images you find attractive.
Wine is an implementation of the Windows API on top of X and Unix. It does not require Microsoft Windows, but can use native Windows DLLs if they are available. It provides both a development toolkit for porting Windows source code to Unix as well as a program loader, allowing many unmodified Windows programs to run on x86-based Unixes.
omega is a complex rogue-style game of dungeon exploration. Unlike other such games, there are a number of ways to "win", depending on various actions taken during play. The ways you can get your name on the hiscore board include becoming the highest ranked head of a guild, sect, college, etc., as well as gaining the most points figured from possessions and experience. The game (via the oracle) may impose some structure on your exploration, but you need not follow all of the oracle's advice. There *is* a "total winner" status, by the way.
The Deimos Project is a 3D space combat simulator and modular game library collection. It includes a 3D modeler designed with a programmer's needs in mind. It allows you to construct 3D models by writing Java source code. All you need to design a model is a computer, some graph paper, and a pencil. The modeler's internals function as a simple OpenGL scene graph API, without preventing the use of low-level OpenGL commands. For storing settings and other hierarchically-representable data, the project provides a Hierarchy library that stores data atomically using XML. It also features a timing library, a game object container system, and many other features.