At the moment in the UK, on top of the cards that britjojo has already mentioned, there is some government sponsored thing called 'funfares' - they are to promote tourism, and are heavily discounted tickets on certain routes. You can travel from pretty much any major city to any other major city for £2 each way (roughly $4). Those tickets sell out very fast though, but there are slightly more expensive 'funfare' tickets available up to the date of travel.
The offer has been running for over a year, and I'm dreading the day they decide to stop it - I've gotten very accustomed to being able to just book a coach whenever I want.