There is a free mod/game on steam named synergy. it's using the source engine to make all the hl2 games and etc. multiplayer, like playing CO-OP on the hl2 story. But it lacks freeman as a playermodel so if I play with my friend we are both rebels. But it would be nice if like I'm looking like Freeman while my friend is the rebel, then it also fit's better to the story mode. I think this would be a popular mod there.