- Putting a working server/client/multitier configuration on a laptop for demoing
- Testing/debugging setup for RegNamespace (the undo disks are a godsend here).
- Doing something sketchy (e.g.. looking at dancingpigs.exe that your friend emailed you)
- Running lots of different versions of an OS, or multiple different OSes at once
That's what I've found it useful for - it's used for tons of other stuff. Read more on the Virtual PC site.
[via Robert Burke's blog]