You can make a bootable version of VMware on a CD/DVD, and run a compressed virtual machine, perhaps using copy-on-write storage, storing your persistent changes on a USB stick or SAN. (I've done it) Great for disaster recovery, just store your VM's on network accessible storage (iSCSI, ATAoE, or even SMB/CIFS) and if a machine blows up, just netboot another machine with a vmware livecd image and boot that VM back up, no messing about with hardware changes.
I'd like to see Microsoft ever do something as close to doing that
VMware Server benefits
* faster than VirtualPC (copy-on-write page migration, custom guest drivers with lower overhead)
* Native filesharing on Linux guests (drag-n-drop)
* USB and SCSI passthrough, connect any SCSI/USB device directly into your VM with no wrapper.
* Scripting API
* Network console access/control (via VNC protocol)
* official Linux, FreeBSD and Netware support
VPC benefits
* hmm.....
* well, you get 'official support' from Microsoft for Exchange 2003 when running under VPC. Now my experience of 'official support' from MS hasn't exactly been gleaming, and it runs perfectly under VMware, but hey.