Here are some links to very useful websites.
The Petrucci Music Library
The most exciting website I've ever found! Download music scores for free. Many of the available pdf's are first editions or autograph facsimiles.
www.pianopedia.com
This website lists works by any composer you can think of, including information about each piece (average duration, date of composition, etc). Sometimes this site is painfully slow, but be patient because it is worth it.