One of my old time favorites when it comes to First-person shooters, is Tribes 1 & 2.
The action is fast, you have different classes of soldiers and it was just pure fun and a hell of an adrenaline rush.
It was not the first fps multiplayer that emerged on the internet, but the reason that I, and many others I know, liked it, was that it was easy to learn, but very hard to master, and you could play for a half hour or a couple of days in-a-row. Lot of maps and a shitload of different strategies made this game a stayer (I even play it these days).
If you are planning a lan party, I think you can get this game in a retail store for a couple of dollars.
Some gameplay from the game