Best streaming software for Twitch

I am going to start a YouTube channel for Lets Play videos and random gaming sessions. I also want to start using my PC to stream games to Twitch. What software is the best for me to use?

Open broadcasting software! Free and amazing.

what ever you do..don't use LTT for advise.. you will end up in HELL. Diver hell- conflict hell, that is. Why their stream is perpetual HELL. But, sorry, Listen to above poster. but beware driver conflicts.. audio shit and aviod SKYPE LIKE THE GREAT PLAGUE OF LOCUSTS. but ti's an neccisary evil. OBS is exellent.. just watchout audio driver conflicts...and remember Skype WIll fuck you evey chance it gets to.