Have a look at sven coop over on steam. It's free to play and coop. Basically, it's the original half-life that's been remade for multiplayer, and it works on really bad pcs (the original half life came out in 1999). I've had fun playing it with my friends, but it is aged. You could try team fortress 2 as well. Some of the most popular MMOs also have very low specs. Hope you have fun.