How can I record all activity on the screen for a certain amount of time? I know about tools like
xwd that capture a still image; can they be extended to record video? I'd prefer not using any fancy debian tools or ImageMagick utilities
You could try xvidcap.
What about a VNC based solution? Start a VNC session (via Xvnc or the builtin gnome/kde VNC tools). Then use vnc2flv, or the older vncrec or vnc2swf.
Another alternative is the open-source Wink (http://www.debugmode.com/wink/). It is nowhere near as functional as Camtasia, but it runs on Windows and Linux. I have used it to create some small recordings, and it works reasonably well.
You can use www.screenr.com which is great for recording screen activity. The down side is that it posts directly to the internet or youtube, and I can't remember if it lets you download a copy of your recording. It requires Java. The quality produced is HD and you can also record with or without audio.
Screenr gives you a maximum of 5 minutes to record. They do have a premium option, though I have not used that so can't tell if that lets you record for longer.