Fraps or dxtory. Dxtory offers much more customization with your codecs but fraps is the simpler of the two.
With fraps, for ever 2 minutes of gameplay at 1600x900 resolution, it takes up 3-4 gigs (File sizes differ depending on how many colors change in your video, so if you record a dark game, it takes up less space).
If you choose dxtory, the Lagarith Lossless Video Codec is the standard (google it).
You may also try x264 codec but I've had trouble editing it with sony vegas (64 bit) and any other video editing software. Also keep in mind that using codecs that compress on the fly will cause your fps to suffer because it needs more cpu power (x264 codec).
From: holysin
camtasia
You can't record a fullscreen game with camtasia (I believe) and the quality suffers greatly. Camtasia is better for recording your desktop.