OPFLASH isn't exactly 'quake-like'. Actually, it's on the opposite end of the shooter spectrum; It's the best in tactical shooters (IE: control your squad, have someone snipe their landed helicopter's pilot, capture heli, empty out enemy base with said heli).
Unreal 2 is a great quake-style game (several unreal 2's, I think :tournament is the other).
Edit: Battlefield 1942 and Red Faction 2 are another good pair to check out.
Edit#2: America's army may be free, but it sure as hell ain't a military simulator.
Operation Flashpoint actually is one(military sim) (theres even an LEO-type version that's a little more expensive and can only be ordered from the dev's website- I think its re-tooled for Virtual Reality play)
Hell, in what other game can you drive a tank, fly a helicopter, steal an enemy truck, and hijack a bunch of enemy supplies all in the same mission?
[url]www.americasarmy.com[/url]
Worth a try- hell, its free (albeit 300 megs)