Excido

Excido is a free (GPL) game. It combines (simple|old school|arcadey) gameplay
with 3D OpenGL graphics graphics and OpenAL positional audio. Excido is not
a highly complex experience, but you may enjoy it as a small diversion.
 
User-unfriendly customization is possible by altering excido.conf, with
reference to keyguide.txt.  The defaults are:

Strafe left: A
Strafe right: D
Go forward: W
Go backward: S
Respawn: Return
Toggle Fullscreen mode: F
Grab/Ungrab input: G
Change camera view: Non-keypad numbers (1,2,3,4,0)
In-game menu: Esc

The mouse is to be used for turning and firing.  Try both buttons!

I would recommend against using the arrow keys for in-game controls, because 
there is some room for weirdness due to the menu system using these keys.  
Basically, if you push a key down, and then bring up the menu, then the 
KEYUP event goes to the menu not the game, and the game thinks you're still 
pressing the key down.  But if you have the presence of mind to just not 
hold down your movement keys while invoking the menu you're fine.

Remember that the excido.conf in your home directory takes precedence over 
the default one that comes in the data directory.

Esc gets you your in-game menu.  Menus are used either with the mouse, or 
with the arrow keys and the Return key.

Gameplay
---------

Shoot the nasty enemies with your rayguns.  Collect the powerups that 
appear as your score increases.

When you die, press Return to respawn.