If you don't want to pay for Fraps and you don't mind messing around a bit to get things working correctly, you could try Taksi. It's gone quite a while without an update, it's persnickety, you'll have to record sound with a separate application (and subsequently mux/sync it with the video), and install a codec to encode to yourself, but it's the only free screencaster that I know of that will hook directly into OpenGL/DirectX like Fraps does.
Failing that and Fraps, I'd use VirtualDub's screen capture.