BZFlag is a 3D, multiplayer, tank battle zone, capture the flag game that pits players against each other in a networked environment. It runs on Windows 95/98/NT/2000, Linux, MacOS 10.x, Irix, Solaris, and others. An OpenGL accelerator is highly recommended, but it is playable with 3D in software.
Synaesthesia is an eyecandy program that represents music graphically in real time as coruscating fields of fog and glowing lines. It is intended as a visual accompaniment to music. Its display combines information about the frequency, location and diffuseness of sound. It can take input from CD, line, piped from another program, or from EsounD. It also functions as a CD player.
Obsidian is an extensible virtual world system with a fullblown multiplayer client-server architecture. It is not a VRML2 world system, though there are plans to include VRML2 as a world data format. Note that active development on this system stopped a while ago, though the code is still available for the curious. The code may not even compile on current Linux or Irix systems.
kicq is an ICQ client specifically designed for the KDE desktop. It has a simple, clean interface very similar to the original Windows ICQ client, and includes all of the most commonly used ICQ features such as messaging, real-time chat, and file transfer. It features complete message history, a New ICQ User registration window, multiple ICQ accounts, SOCKS5 proxy inline support, configurable sound events, Cyrillic koi8-r/cp1251 recoding, and support for the ICQ UDP v5 and ICQ TCP v2 protocols.