The Neue Mozart-Ausgabe is by far the best online resource for all of Mozart's works in a scholarly edition.  The site states that usage is restricted to "personal study, educational and classroom use".

The entire collection can be searched by category of work, a variety of KV. catalogue numbers (KV, KV6, KV6 Anh. A B or C...who in the world knew that so many librarians had so much time on their hands?), and even by key signature and preferred editor.

Here's a look at the Mozart Piano Trio in B-flat Major, KV. 502:

Pages can be viewed a portion at a time by scrolling down the sidebar, or by clicking the page number hyperlinks along the top.  Beautiful typography makes viewing easy on the eyes and friendly for study.  Kudos to the Internationale Stiftung Mozarteum and The Packard Humanities Institute for making this treasure of musical art freely available to the world!