Is there any free screencasting software?
I don't do a lot of LPs, but the last time I did I used QuickTime Player.app (Quicktime Player X); you'll find it in /Applications. To record your screen, select File > New Screen Recording.

The results were pretty good; recording Minecraft 1.7.3 gameplay on an about a year and a half old MBP, but I have a lot of RAM, so that certainly helps.

Edit:

It's worth noting that QuickTime Player X does not support loopback sound; meaning that the only sound recorded will be from the microphone, or virtual sound device providing loopback sound.

This may or may not be a good thing, depending on what you are doing.