Or better yet: xyzzy.stackoverflow.com to test against?
ETA: sandbox.stackoverflow.com is a much better name, and better encapsulates my idea. Thanks dbr.
There has been times I'd want to try something about the StackOverflow site without touching the actual site. I can't remember specific examples, but one would be deleting my own answer when a bounty is issued, or how negative reputation is handled etc..
The site could be reset every 24 hours, perhaps even users (with over x reputation on SO/SF/SU) could create (temporary) accounts with specific privileges/reputation.
The FAQ and sofaq tag has satisfied most of my curiosities, but it could prove useful..
I would actually like to see something like this for new moderators. There are numerous new tools available, one of which is recalculating individual users' reputation. As a test, I recalculated mine and I lost the 99-point rep boost that everyone has when they start here. I emailed Jeff to see if he could restore it, but I haven't heard back.
There are also some other tools that I'd like to try out (like deleting/destroying users) but I don't want to junk up this site with test accounts.
Meta Stack Overflow is the testing site / sandbox.
While it might be occasionally useful / amusing to have another one, in practice I don't know that it would enable much beyond what is already possible here.