recordMyDesktop and frontends
recordMyDesktop has both a command-line interface and two frontends, a GTK and a Qt graphical frontend. You can run it in a console, do whatever stuff you want to record, then hit Ctrl+C to stop it and recordMyDesktop will create an out.ogv (Ogg Theora) video file in the current working directory.
recordMyDesktop - GTK interface
If you need to customise the video settings for the screencast, you can do it either with the command-line tool or by using one of the graphical frontends. For example, to run the GTK application use gtk-recordmydesktop.
recordMyDesktop will allow you to create a screencast and choose the video and audio quality, frame rate, include or exclude window decorations and the mouse pointer. It's easy to use and will surely do its job in a few clicks.
XVidCap is another pretty good GTK-based recording application. The first thing which jumps into attention when starting XVidCap is a red rectangle which can be moved around and resized and which will allow you to record only a certain portion of the desktop.
XVidCap allows you to save the screencast as an MPEG or AVI format.
These are not the only applications with which you can make a screencast. Have a look at Istanbul too, which is included in the repositories of all the popular distributions. Although it is probably the easier to use of all three, I found Istanbul to be the slowest when it comes to response time (it looks like it takes forever to encode the video), so you'll have to bear with it. Just like recordMyDesktop, Istanbul saves the screencasts into the free Ogg Theora format.
Have some other cool applications or ways for creating screencasts? Please share them in the comments below.