Running VMware Server 2 (with VIclient interface - none of that horrible web interface for me!) for a few reasons:
2 Ubuntu 8.04 VMs for folding
1 Ubuntu 8.10 VM for simulation development
Other misc VMs for trying linux OSes (new releases etc)
This machine needs more RAM (plus a dual socket i7 upgrade methinks)! I tried hyper-V, but needed CUDA support on the host OS (server 2008), the driver for which made everything super slow. Hyper-V is great - seemed a lot faster at booting VMs than VMware server, plus the 4 way virtual SMP is great. Realistically though, i want to use my desktop a bit, so server 2008 had to go!