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.
simpleJ emulates a simplified computer on your PC: a retro-style video game console that lets you understand how a computer works and write your own video game programs. The first microcomputers were quite simple. They had a few kilobytes of memory and couldn't run large programs. Many didn't even have an operating system. Yet they had something valuable: understanding everything about their hardware and software was easy. simpleJ aims to make this possible while running on modern PC hardware.
Vectoroids is based on the X-Window PDA game 'Agendaroids,' but uses libSDL for graphics, sound, and input, and includes nicer graphics, sound effects, and music. The objective is to survive wave after wave of giant rocks, which you must shoot to break into smaller pieces, until they are completely destroyed.