above and beyond what the mtgox web site offers, it supports placing a "batch" of small trades across a spread of prices, a "cancel all trades" button (which is really handy during an unexpected market shift which conflicts with your trades), a built-in market chart and list of live trades, and a much faster UI than the mtgox web page offers (no waiting for pages to load, list of trades to load, etc).

It uses the HTTP api to place trades and the socket.io api for market data, making it use-able even during most DDos attacks when the mtgox web page is down or so slow that it's un-useable.