Open Broadcaster Software
I've had real hit-and-miss (mostly miss) results when it comes to trying to stream to Twitch from my AverMedia Live Gamer Portable (which works really well otherwise), and was considering upgrading to an Elgato Game Capture HD60 but a few YouTube videos have suggested similar streaming issues when attempting from a Mac.
However, some have suggested Open Broadcaster Software as an alternative to the apps that AverMedia and Elgato provide. It sounds like people are getting better results and less headaches when using OBS, and so I'll be trying out the software very soon.