A shell script which can be used to automate the backup of virtual machines running in a XenServer environment. It's specifically written for XenServer v5.5, and I think it'll run reasonably well on top of XenServer v6.1 (I THINK).
A Perl script which analyzes the OS it's running on top of to determine whether or not it's virtualized, and if so which product(s) it's inside of. Uses multiple techniques (no red pills, I don't think) to gather information.
A VPS hosting provider that uses Xen and lets you run BSD as well as Linux. Could be useful.