Current State

The strong presence of Linux in the domain of Unix is not to ignore. However, there are a couple of alternatives, as FreeBSD, GNU, IRIX, Mac OS X, NetBSD, OpenBSD, Solaris, and more, but Linux is definitely the most popular.

Naturally the dominance of Linux makes it the most interesting Unix alternative for game developers, and indeed games, libraries, engines, tools do exist. Before any progression, it is wise to make some research about the situation. Google gave me for:

• "game development" - 1.820.000 results,
• "linux game development" - 9170 results,
• "unix game development" - 3 results,
  

A clear statement, right? Well, not for me. I have to see the results myself, and make up my own mind.

• Linux Game Development Center (Website)
  
• Simple DirectMedia Layer (API)
• Allegro (API)
• ClanLib (SDK)
  
• Mesa (OpenGL)
• Programming Linux Games (Book)
  
• Linux Game Programming (Book)
  
• Linux 3D Graphics Programming (Book)
  
• Advanced Linux 3D Graphics Programming (Book)
• Open Source Game Development (Book)
  
• OpenGL Programming for the X Window System (Book)
  
• The Linux Game Tome (Forum)
  
• X11 Game Programming (Tutorial)
  
• GIMP (2D)
  
• Blender (3D)
• Inkscape (Vector)
  

This list might not be complete, but there isn't too much left. Don't underestimate these resources, they are pretty fine. It's a pitty having so few die-hard games using them.