fear combat (which, basically, is the multiplayer part of fear) is free. You can also download the "coop warfare" mod, which, you guessed it, allows coop play.
Technically System Shock 2 is probably not free, but still readily available for download. This is noteable because there is a highres texture pack and a coop mod available, too. Getting the thing to run can be a bit of a pita, but it's very much possible. You have to edit a few things in a config file, I cant remember which offhand, sadly. The coop mod has a few bugs, usually you overcome them just by quicksaving and reloading on the spot. Keep a few major saves, just to be sure. In the end of the game, where you have to use missiles as ladders in one room, there's a bug that crashes the game. Easy to avoid, tho. just have player 1 finish that section, while the other players just wait in the room. it's only 20 secs to the level exit from there, really no biggie. I'd say try it, it was a great LAN experience for me.
IIRC you can get IL-2whatever Sturmovik for free. With full realism I really enjoyed dogfighting.
Maybe Trackmania Nations? I dont like it as much, because racing cames without collisions between cars might make sense, but take the fun out of LAN evilness.