Here are three great tools for recording screencasts on GNU/Linux:
Byzanz is a command-line tool. You can set options like the duration and delay of the recording. It can record into GIF or OGV/OGG formats.
Istanbul is a GUI-based tool. It sits in your tray and all you have to do is click on it to start recording. Click again and it stops recording and you can save the video.
recordMyDesktop is a great little tool that has a GTK and Qt frontend.