UBS, the financial entity, has a nice banking dictionary on their website. It is currently in the 2007 edition. It contains over 2400 terms and is a very simple interface. It is one of the most comprehensive that I have seen online. Just choose a letter from the navigation list on the left side of the page.
Another option is the dictionary produced by the Comptroller of the Currency, which is part of the Department of Treasury. One more stop might be the Federal Reserve Bank of Chicago, which lists several useful financial and banking glossaries.