IMSLP (International Music Score Library Project) started in 2006 and is one of the largest online digital repositories for music in the public domain. Since its creation it has amassed a collection of over 420,000 scores, 48,000 recordings, and 15,000 composers.
While most commonly used a resource for quickly accessing orchestral parts and scores, IMSLP is a great tool for viewing a composers complete list of works. IMSLP also can be used to discover a composer's solo and chamber works for your specific instrument.
On the site, users can find a variety of musical scores, parts, solos, and audio recordings. It should be noted that the IMSLP database is primarily represented by music of the Western tradition, and those in search of works from outside this realm will have significantly fewer options. Scores by European composers from the 18th and 19th centuries are the most common, while works by those from early 20th century slowly become available each year due to expiration of copyright. Interestingly, the IMSLP has recently become a more attractive depository for newly written compositions. Using the Creative Commons license, a great number of 21st century compositions have been uploaded by the composers themselves, thus using the IMSLP to sidestep traditional methods of publication.
IMSLP is a unique database. It is built like like a Wikipedia page, but with the functionality of a Google search engine. A sample search for "trombone solo" might yield a page like the image below. Keep in mind that these are only listed pieces in the public domain, meaning generally pieces from the last 50 years are not going to be available. Also, IMSLP may not have scores or recordings for more obscure composers or works because they are generally harder to access and have not been scanned into the database as of yet.
Clicking on a specific piece brings up a page like this, which contains the metadata and actual file:
