Red Orchestra. I own it and love it. Not n00b friendly and can take a little while to adjust.
The most compelling WW2 fps I have played as far as multiplayer goes. Master the bolt actions as fast as you can because you'll be spending a lot of time with them. It is about as far as a game can go as far as realism goes, but it doesn't have backfires, jams, bullet penetration or dud grenades. Make exstensive use of cover or the snipers/bolt actions will make mincemeat of your arse. Most of the maps are quality, some can be a bit boring. It's very well balanced on the maps where it is meant to be. Not all WW2 battles were balanced and the game does reflect that, particularly in some of the 1941 era tank maps, with the Germans wiping the floor. Highly atmospheric and pretty tense at times. A decent pair of headphones or surround sound will make the arty bombardments a pretty awesome thing to cower under. You need a decent bit of patience and situational awareness. Sharp eyes help. Don't immediately try to take the sniper rifle as it is the only real counter to arty in the game and you can piss off a lot of people.
Absolutely brilliant. Absofuckinglutely brilliant if you can find a good server that is regularly populated. Why don't I play it? Because there aren't many Aussie servers and they only used to have anybody on them on Sat night. The community is mostly older dudes, and includes some history buffs. Pretty good bunch in general. It's a pity it didn't have the hype machine behind Halo3 backing it. With the right exposure it could have been as big as CoD1 and 2 for MP. Or even BF2.
This is only a personal opinion though, the game will definitely not be everybody's cup of tea.
Whoa... Can't believe these forums are still kicking.