Hit by a car, rear wheel buckled

or if you are feeling handy, buy a spoke key for a few quid and look here:

http://www.bikemagic.com/maintenance/wheel-truing-basics/3041.html

just turn the bike upside down and use it as a truing stand ;)

+1 And use the brake pads (unless brakeless) to gauge true.
It's very satisfying knowing you can do something that many people are scared to even try.