I second ARMA, whether it's the first one or the Uber-realistic ARMA2. I would recommend the Brothers in Arms series if you like WW2. Also, since you didn't specify type, I'd suggest a look at Company of Heroes and it's expansions. Extremely fun and realistic RTS based on WW2. MOH Airborne was a great game too.
If your willing to try something outside of FPS.. I'd recommend Soldiers: Heroes of World War 2
Its an awesome game that combines alot of different elements and has a ton of detail. Hard to describe the gameplay really.. the main feature of it is you control a set number of units and have to take an objective which usually has different means of doing it and the unique part of this game is you can individually control one unit to get better aim using your mouse (still keeps the top-down view though) and use WASD to move around so its sort of a hybrid of a FPS and squad-based game. The level of detail in the game is insane vehicles have fuel and ammo (which can be salvaged from vehicles you disable but not destroy or repair it throw some guys in it and use it yourself) multiple hit points on vehicles so you can disable the treads on a tank to prevent it from moving and then circle around behind it to destroy it where the armor is weaker. Lure enemies into a wheat field then set it on fire.. throw a molotov onto an APC and watch enemies run out of it on fire.. grab a minesweeper from the back of a truck and then head to a nearby minefield to pickup mines and place em on a patrol path of an armored convoy etc.
There is also a good amount of variety in missions from tank battles to stealth missions that rely on prone and using cover to prevent enemies from being alerted to you and calling in backup.
I always wanted to play that. I passed the first time around because the hardware requirements were too steep for me at the time. Maybe I'll see if Steam has it and take a look. I played the demo a long time ago and I remember the direct control feature. Pretty cool idea. Wish more games used that. Speaking of direct-control-hybrid-genre games, I would really love to see another Battlezone game come out. I was addicted to the first two. We need more FPS-RTS blended games