American law once required works to be registered with the Copyright Office in order to obtain protection. While that is no longer the case, works are registered with the Copyright Office because registration provides benefits to the copyright owner. It also provides constructive notice to infringers that the work is copyrighted.
The Copyright Office has an online database that includes information on new and renewed copyrights filed from January 1, 1978. Information on older copyrights was published in the Catalog of Copyright Entries (CCE). All of the volumes of the CCE have been scanned as PDFs. A listing of those volumes is maintained here by the University of Pennsylvania Library. Their list links to the scans of the volumes in various online digital repositories.
For the ownership of musical compositions, there are several online databases that can help. Among these are the Copyright Office's registration records, the SongFile service of the Harry Fox Agency, the ASCAP and BMI combined Songview repertory database search, BMI's repertoire search, or SESAC's repertory search.
The online database Writers, Artists, and Their Copyright Holders (WATCH File) is a directory maintained by the Harry Ransom Center at the University of Texas at Austin and the University of Reading to identify owners and contact information of copyright holders of literary and artistic works. For example, it reveals that Sylvia Plath's copyrights are owned by her estate, which is represented by the London publisher Faber & Faber.
Artstor is a non-profit organization that licenses photographs and images of art, primarily to schools and libraries. They have a large database that can help locate rights holders. The Artists Rights Society (ARSNY) represents thousands of visual artists including Andy Warhol, Robert Indiana, and Pablo Picasso and can identify owners and issue licenses.