Hi john_sm, i'm quite new in virtualization, but actually I'm using vmware server (it's not open, but free to download and use) with windows 2003 server. Over it, i virtualize a cluster of two web servers (debian 4.0r1 with apache+php) to try high availability with heartbeat and two database servers (debian 4.0r1 with Oracle 10g XE). Performances are good and the virtual machines creation is fast because I've created a single virtual machine (a kind of template), then I've cloned it several times. In this way I don't waste my time in building new machines from zero. Also, if I made mistakes with a virtual machine, I can destroy and replace it with a new one in just few minutes.
About XEN, I know it's a software for para-virtualization that consist in a software layer that besides immediately over the hw of your real machines. It runs only on linux and can virtualize only linux-like systems.
Virtual box is an open source solution for virtualization, but it's used frequently by desktop users, for example to try new linux distributions without installing them (I don't know if virtualbox it's used seriously also in server and production environment.
Finaly, there's virtual pc by microsoft, but I've never used it.
So, there are many solutions for virtualization, but depending of your needs.

Simone