I'm fed up playing FIFA 11 with my brother, so I need some 2 Player Split Screen or Co-Operation PC Games now. Would love games, in which both of us can work together, rather that being against each other. Something like Contra that people used to play in PlayStation. But others will work too. Don't really like plain racing games. Something like Burnout Takedown or Hot Pursuit is more preferable. But Takedown isn't for PC.
Trine is a good one for co-op. 2d platforming. Available on Steam or GamersGate (probably other places too).
The co-op mode on Portal 2 is good, but I'm not sure if the PC version includes split-screen.