FreeBSD has Jails, which is basically OS-level virtualization (the original inspiration for Solaris Zones).
This functionality is built in to the operating system, and wouldbe "os-level virtualization" (in the sense that you can run several 'virtual' FreeBSD systems on one host).
There are tools in the ports collection to manage/work with jails which implement varying degrees of management on top of the basic
jail subsystem - lots of articles/blog entries around the net have been written on these tools and will give you a good idea of how they work / what they can do, but there's no substitute for installing one and banging around.
As far as I'm aware there's no true (bare-metal simulating, VMWare-style) virtualization based on FreeBSD. You can hack something together with QEMU/VirtualBox, but if you need mutliple-OS bare-metal-simulating virtualization you're probably better off with VMWare's ESXi (free or fully-licensed depending on your needs), or Microsoft's Hyper-V platform...