You can do this with Google, too: http://google.com/hotels

1. In the "search for a city, address or hotel" put in the name of the BART station, e.g. "Powell St. BART Station, San Francisco"
2. Pick your dates
3. Find hotels on the map in the panel on the right or the listing on the left

It will even tell you whether the rate is more or less than normal, and which booking site has the lowest rate.