You could use gamecam. There is a trial/limited version, but it works in most games. Unfortunately, I could never get it to work with TF2. There is also the problem of it showing what's being recorded in the bottom right corner, so it obstructs your ammo count. It's not very high quality, but it gets the job done with relatively minimal disk space. It also has a watermark that can't be removed unless you buy the full version.
Fraps also has a trial version, but that one is limited to 30 days. It has pretty crystal clear quality, but it does have an unremovable watermark for those using the trial. It also takes a lot of disk space on higher resolutions. It also likes to cause slowdowns in FPS rate. To specifically counter this, I went out and got a 9600 graphics card. Not the recommended way, but I wanted a new graphics card anyway. I mean if other people are gonna be watching, it's nice to really show off the graphics. So in example, if you run a 1280x1024 resolution in-game with 60 FPS at half-size, you start running around 1 GB for 10 seconds of footage. If toned down to say 30 FPS, you can go about 30 seconds for 800MB. Just fiddle with the settings on Fraps and in-game until you get something you can accept.
You could also use the in-game recording, but I'm not sure how to work with/edit that.