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.
Kobo Deluxe is an enhanced version of Akira Higuchi's game XKobo. It adds sound, smoother animation, high resolution support, OpenGL acceleration (optional), an intuitive menu-driven user interface, joystick support, and other features. Kobo Deluxe uses SDL for graphics, sound, and input, which makes it possible to run the game on a very wide range of platforms.
Deliantra is a client for Deliantra game servers. It offers an intuitive user interface for exploring the vast game world, a persistent minimap, powerful macros and keymaps, dialog-based NPC interaction, and easy configuration for the skill, spell, and item system. It is based on Perl, SDL, and OpenGL for high performance on modern systems, and works on both Unix and Windows. Ports to other platforms should be very easy and are underway.