You used to have free and searchable access to federal district court cases only on a district by district basis, but the situation is improving. Librarians and lawyers have for years been arguing that these are government funded databases and access should be available for all for a nominal charge or for none at all.
LexisONE® has offered free case searching for a long time, but neither they nor anyone else offered free federal district court case searching until now. JUSTIA has launched a searchable database of just these cases. The database is new and not all the bugs have been worked out (it’s free, folks), and is worth a look-see. Visit Justia or go directly to their list of cases databases at cases.justia.com/.
There are LOW-COST legal research databases for those who want and need a bit more control over your searching options. A guide to these low-cost legal databases is at the University of Washington Law School Library (and say thank you to the law librarians for all their excellent guides.)