Virtualbox is a open source virtualization tool that runs on many operating systems, including Mac OS X It's free, simple and it works, but you'll need to be aware of how to use it. Here are my experiences with different settings.
The default network driver makes an airport interface (en1) on Mac OS X unstable. So; either map the virtual interface to a wired network interface (en0) or use another driver. The Intel PRO/1000 MT Desktop (8254OEM) works fine for me, the PCnet-FAST II (Am78C973) does not work well for me.
For FreeBSD guest operating systems, the default IDE disk causes timeouts. Better use a SATA emulation, IDE emulation causes errors during the install.
Have fun using Virtualbox, after all, its free!