I've also made, a website, which I personally use for small arbitrage opportunities.
http://bitcoinprices.comBesides MtGox and Britcoin, there is also Bitomat. Hopefully soon I will also add Bitmarket. Right now one has to simply switch between markets, to see their current exchange rates and price differencies. Probably I will have to add also something, that will instantly recalculate and easily show the possibilities for arbitrage. Definitely this is an interesting topic.