If you're on a budget trip, then staying overnight at a transit airport can sound like an appealing prospect. Sleepinginairports.net offers an airport-by-airport guide to whether you can realistically plan to kip at your interim destination.
Airport policies vary widely when it comes to tolerating sleepers (you can't move at London Stansted at 3am for people sleeping before early flights, while Sydney's domestic terminal actually kicks people out), so checking out the airport reviews at the site is definitely a sound idea. There's also some useful general tips on how to plan for an airport stopover, including such minor gems as travelling with Vicks so you can block out unpleasant terminal smells. Thanks Alex!