Armed Assault 2 (ARMA 2)

Support is native for 16:10, 16:9 and 12:3.
From the Video options menu, choose your desired resolution (Interface resolution) then select the corresponding aspect ratio in the Advanced menu. Restart the game to apply changes.

However for proper/unstretched 15:9 and Surround, a simple edit is required.

1) Find your .ArmA2Profile file in My Documents\ArmA 2\ and open it in Notepad.

2) Look for these lines:
fovTop=
fovLeft=

3) Edit these values to match your aspect ratio as follows:

For 15:9
fovTop=0.666666666;
fovLeft=1;

For 3x 16:10
fovTop=0.8;
fovLeft=4;

For 21:9
fovTop=0.94;
fovLeft=2.1900001;

4) Save changes and launch the game. Your new AR is automatically selected and listed as 'Custom' in the Advanced video options.