I use VirtualBox fairly regularly for virtual machines of various types (XP under Debian, FreeBSD, etc) on my main desktop. Compared to VMWare I have no complaints. It works flawlessly, and with the latest release things are just polished.
Also, it now supports OpenGL for guest VMs. This is huge for graphics work / gaming, etc. No more dual-booting. With RAM as cheap as ever – there isn’t a reason why VMs don’t fit the bill for servers anymore.
Lastly, but more importantly, VirtualBox is free. Yes, free. It’s a quality product, and really meets my expectations for virtualization.