HeinOnline is building a big digital collection of Congressional committee hearings in its U.S. Congressional Documents Library (U.S. Restricted).
The number of hearings currently available is more than 3,200. These resources come from Covington & Burling's hearings collection, which covers from 1927-2001 (71st-107th Congresses). Hein will harvest hearings after 2001 from the U.S. Government Printing Office's FDsys site.
Hein plans to add a thousand hearings a month, leading to more than sixteen thousand hearings and more than 5 million pages.
The hearings collection is a wonderful complement to the digitized Congressional Record already available on HeinOnline.
Congressional committee reports are available online via the Readex U.S. Congressional Serial Set, to which the UW Libraries subscribe.
All three of these sources--hearings, reports, and the Record (with floor debates)--are essential for researchers investigating the legislative intent behind enactment of a federal law.
See the Gallagher guide on Federal Legislative History for more information.