Screen recording utility

[Daniela]

I'm looking for a tool to record everything that can be seen on a given X 
display. Ideally, it should have support for limiting recording to a single 
window, but that's not strictly necessary. About four years ago I used 
Camtasia on Windoze for this, and I need something that produces a similar 
result, preferably a neat small command line tool.

If such a thing does not exist, can someone please give me a hint on how I 
could do it myself in ASM/C/C++/shellscript/whatever? Is it as simple as 
reading from a device file, or can I just put this feature into the X server?

Regards,
Daniela