Quake III Arena

1) Go to <gameDir>\baseq3.

For Quake 3 Arena purchased via Steam, the correct config file should be:
Steam\SteamApps\common\quake 3 arena\missionpack\q3config.cfg
NOT: Steam\SteamApps\common\quake 3 arena\baseq3\q3config.cfg

Edit q3config.cfg (preferably with textpad) and edit these lines:

seta r_mode "x"
Set this to -1.

seta r_customwidth "xxxx"
seta r_customheight "xxx"
Set this to your favorite widescreen resolution.

seta cg_fov "90"
Set this according to your preference. I recommend 100 for 16:10 resolutions and 105 for 16:9 resolutions.

2) Quake 3 Resolution Changer