Debian includes VirtualBox-OSE in its repositories, with pre-compiled kernel modules, however I opted to go for the official Sun binaries.
Read it here
The main thing I use Virtualization for on my Linux systems is to virutalize Windows XP. I dont do a lot in XP, but one thing I do is connect back to my workplace via VPN. I know pptp VPN can be configured in Linux, but even when I was running Windows as my host OS, I always used a Virtual Machine for VPN for a few reasons. First, my VMs dont do much – I know they are clean, and I feel safe attaching them to my workplaces network. Secondly, creating the VPN tunnel in the Virtual Machine doesnt disrupt my local internet connection.