Kon-Boot looks like a very interesting tool since it can get you into a system without having to logon first.
According to the description at the tool's site, Kon-Boot alters a Linux or Windows kernel on the fly during boot up. The result is that you can login to a system as 'root' or 'administrator' without having to know the associated account password.
The tool reportedly works with Windows Vista, XP, Server 2008, Server 2003, Windows 7, Gentoo, Ubuntu, Debian, and Fedora.
All of you admins out there might want to give this tool a whirl to see how it works against your systems - before one of your users does!