I use ScreenFlow and have been very happy with it. It's not the cheapest at $99 retail, but has all the bells and whistles I have ever needed while creating tutorial videos.
I tried QuikTime but that didn't work out very well. I'm trying ScreenFlow now but it creates lag in the game for me... Will try Viola and Copernicus when I get the chance, thank you!
i have been using the free version, which sadly only gives you 15 seconds at a time and you get their logo on it too, but works in a pinch: www.screencast-o-matic.com/