There are increasingly a ton of scores online. Some advice/warnings:
These resources help you organize and collect records (whether you want to purchase them or just learn more about them) and give access to liner notes, recording details and more.
Discogs is a crowd-sourced site with good moderation and excellent metadata capability. Concept of "master release" is very good at grouping reissues on different formats. With content driven by users, however, the amount of detail varies greatly, and usually limited to what is physically found on the item.
Open-access database of music information, growing steadily, and integrating RDF concepts, utilizing Linked Data, and offering easy integration into other applications.
Their Picard software is amazing at identifying digital audio and tagging it, if the release is in their database... Tons of potential, but still growing.
All Music Guide migrated online more successfully than any other print publication. Great for all sorts of information and quickly seeing releases and credits. For newer releases, will often have more complete and well-organized data than any other site.
There are plenty of decent online music shops, but these tend to have the largest catalogs and best data.
Extensive database of the major European presses with sample pages, good bibliographic data, and a relatively robust stock.
The Amazon of sheet music, carries the largest inventory in the United States. Database is not always up to date with latest ISMN numbers, and search engine tends to sort by popularity, so extra keywords are a must to keep all the "100 Favorite Classics for Piano" from being your top result.
Primarily serves libraries, so has an extensive database of scores, and helpful ways to search, browse, and filter. Does not keep a large inventory, however, so not for a quick purchase.
Primarily used as a way to facilitate rentals and requires a free account to search. Very useful for quickly seeing what is available and the current publisher.
Fantastic vendor for classical recordings, with easy to use browse interface, compilation of reviews from multiple sources, and data on recordings. Licenses out-of-print items and rereleases them. Not comprehensive by any means, but a great place to start.
Another very good vendor with detailed information on releases and excellent metadata control. Also offers audio samples and Hi-Res downloads.