It's rare that I read anything on the old intraweb that makes me hoot out loud with laughter, but Chris Rae's English to American dictionary is absolutely priceless. He's got a cracking sense of humour, meanwhile maintaining a useful web resource. Go on: buy him a pint for his efforts.
I also hadn't quite realised how much English-English-specific slang I used myself on a day-to-day basis. It's no wonder no one seems to understand a word I say over here.