Now that Steam's all couch-friendly thanks to Big Picture mode, it'd be nice if more games were reconfigured for slouching. Oh, what luck! Just after I write that sentence apropos of nothing, I notice that Valve has released a Portal 2 patch enabling split-screen co-op in its puzzle 'em up.
Portal 2's console edition came with split-screen co-op options but it was never officially supported on PC. Cunning fans had managed to tweak and cajole the game into supporting split-screen, but poking around in config files is not for everyone. Now, all you need to do is play with two controllers in Big Picture mode, which is easy enough.
The patch notes have a little more information, and details two fixes too.