From the webpage:
The Open Data 500, funded by the John S. and James L. Knight Foundation and conducted by the GovLab, is the first comprehensive study of U.S. companies that use open government data to generate new business and develop new products and services.
The full list is the most likely to be useful resource at this site. You can filter by subject area and/or federal agency that is supplying the data.
Great place to look for gaps in terms of data based products and/or what areas are already being served.
I first saw this in a tweet by Paul Rissen.