Open Jurist Offers F. 1st, 2nd, 3rd & US Reports
Open Jurist is a new (I think) database of free US Supreme Court and Federal Appellate opinions. It joins a number of other sites which offer federal opinions, such as Altlaw, PreCYdent, Public Library of Law, Justia and FindACase.
However, the coverage for Open Jurist extends farther back in time than do the others. It contains U.S. Reports from volume one and the Federal Reports back to the first series.
To help you navigate which content is available on what database, you might find the UW Law Library's Guide to Wisconsin Legal Information Sources to be a useful resource. The guide will also soon appear in The Wisconsin Lawyer.