Hello *nixer,
I was watching a nice conference yesterday about jailkit.
It's really something you should check out.
The conference is mainly about BSD systems but it can also work on a GNU/Linux box.
For those asking, Jail is a OS virtualization system, a simple and efficient one.
It can also be used for various purpose.
I am not going to start a huge discussion about jails because it can become boring... everything is in the man page of the package jailkit.

VIDEO : http://www.shmoocon.org/2006/videos/Ike-Jail.mp4

Hope this can be interesting for some people.
EDIT: yrmt already had a thread about jail but the video is still very interesting to watch.