Free Sheet Music
Imslp.org The sine qua non of all free classical music, IMSLP now has a “search by melody” linked section and categories by genre, era, and instrumentation. Various legal challenges have been brought in the past against IMSLP’s public-domain collection, but the site is still going, and stronger and more useful than ever.
http://www.music-scores.com/useful collection featuring Sibelius Scorch playback and score previews, with music for all skill levels drawing on the classical canon, unusual pieces, and new compositions. Some scores require a subscription to print, but the playback section is always free.
https://urresearch.rochester.edu/viewInstitutionalCollection.action;jsessionid=BC940326E443F1ABB943E6C46A04863B?collectionId=25 The Sibley library at U Rochester has a great database of scanned music (i.e. no Creative Commons work) and is especially useful for piano repertoire and turn-of-the-century works. Much of their collection is now duplicated to IMSLP, but the number of unique holdings at Rochester are still worth searching if IMSLP doesn’t turn up enough for you.
http://www.mutopiaproject.org/browse.html#byComposer All typeset using Lilypad, Mutopia’s various works are fewer and more mainstream than the others mentioned here, but the editions are cleaner and nice to play from. Sometimes an odd gem will appear, so I check the new listings every so often.
CPDL.org Good choral music site, wiki-based for ease of use. Most of the collection is cross referenced along with IMSLP at Wikipedia, so if you look at a composer bio linked from IMSLP, you might find a different set of results at CPDL.
http://icking-music-archive.org/index.php The Werner Icking music collection is also linked frequently at Wiki, and generally is good for Baroque or organ repertoire.
Honorable mentions:
The Smithsonian collection and the National Library of Australia both feature limited full-color scans of 20th century editions, generally popular prints but also some lesser known music. These sites are good as reference, but not for printing or downloading.
Recordings
Classiccat.net is great for downloading free classical music recordings. This site does not host any recordings, but archives them from around the web categorized by composer. The quality can be hit and miss, and the same for the coverage overall for each composer, but I've found some gems here as the sites referenced often contain large, quality collections and other links.
Naxos Music Library has an extensive listing of labels and records both famous and truly obscure, all worth sifting through and updated frequently.
Classical Music Library and Classical Scores Library are two halves of an excellent archive which fills in important gaps in Naxos and IMSLP's collections. The recordings at CML are not as many as Naxos but I have often found recordings there of artists not listed with Naxos, or historical records from an era not yet available to stream at NML. The scores database in some cases extends past 1923, so the scores are not always public domain; the downside is that you cannot download them (though printing is allowed and if you have a "print to" option such as PrimoPDF you may be able to download a copy in that manner).
Naxos Music Library has an extensive listing of labels and records both famous and truly obscure, all worth sifting through and updated frequently.
Classical Music Library and Classical Scores Library are two halves of an excellent archive which fills in important gaps in Naxos and IMSLP's collections. The recordings at CML are not as many as Naxos but I have often found recordings there of artists not listed with Naxos, or historical records from an era not yet available to stream at NML. The scores database in some cases extends past 1923, so the scores are not always public domain; the downside is that you cannot download them (though printing is allowed and if you have a "print to" option such as PrimoPDF you may be able to download a copy in that manner).
Academic Research
These are the databases which my school subscribes to, but some of their functionalities are available for general use.
EBSCO I generally use this database to collect the first few articles on my topic, because EBSCO covers a wide range of topics and levels of focus (i.e. scholarly articles, opera reviews, newspaper editorials).
JSTOR‘s collection overlaps with EBSCO but contains many unique periodicals and subject coverage. Large full-text collection available.
Project MUSE Another full-text database, MUSE is for the arts and offers citations and downloads for their collection.
Grove Music This database covers the serious, in-depth articles of Grove and the quick-reference entries of the Harvard and Oxford Dictionaries and the Oxford Companion to Music. Also available at this site is Richard Taruskin’s Oxford History of Western Music, which is good for a long read or a reference for what you know and should know on the subject.
