VirtualBox is probably the most friendly if you only need to run 1 or 2 VMs at a time.
If you need to simultaneously run many different Linux environments (10 or more per host) then OpenVZ is the way to go. It's like chroot but provides completely isolated Linux environments (containers) with the ability to control resources, do check-pointing, and live migration. I'm using it for already more then 2 years for many different sysadmin tasks at work (a 400 user Bioinformatics center at a large university).
OpenVZ has almost no overhead. It's the only one of it's kind (operating system level virtualization). It handles well Linux applications of any proportion from a web reverse-proxy to an I/O intensive backup system processing 30TB a day. Having 30 or more containers per server is normal. Another big advantage is that from the hardware node (equivalent to Dom0 in Xen) you have all the file systems of the Linux containers mounted directly - no NFS required. Also, you can see all the processes of your Linux containers from the head node with the ability to strace, kill, etc...
You can safely delegate containers with ssh access to your friends and let them be root.
You would need to be comfortable with Linux and the command-line. Being able to edit start-up scripts would be helpful (quickly give you a lot of control). For more advanced setups, may need to learn some networking.