Classical Vocal Reprints
An American site with a huge selection of scores available to purchase and download, including seriously obscure titles, many of which are out of print and unavailable elsewhere. They also run a separate site where you can purchase printed scores for mail order.
French Baroque Vocal Music scores
French musicologist Nicolas Scéaux offers freely available edited scores of works by Lully, Charpentier and Rameau, with orchestral parts and even links to the sources.
This promises to develop into a really useful resource for singers and their accompanists who have limited patience with IMSLP's Chrysander editions of Handel's vocal works, tenor clefs and all. Christopher Sokolowski has prepared transposing vocal scores of a number of Handel arias, which are available to purchase, and invites commissions for further additions to his catalogue. His site also includes repertoire lists, and a dramatis personae of many of the singers for whom the roles in the operas and oratorios were written, very useful for anyone looking to expand their Handel repertoire.
International Music Score Library Project (IMSLP) / Petrucci
Since 2006, this Canadian-hosted site has been building up an online library of public domain music scores on the wiki principle of community contributions. Hundreds of thousands of works, by thousands of composers, are now available here and free to download. The only real restriction on the site is copyright law: the works of composers who died less than 50 years ago are still under copyright in Canada and therefore cannot be published here (in the EU, the copyright period is 70 years). This isn't the place to go for the latest scholarly editions, but as a means of getting instant access to a vast wealth of music, it's hard to beat.
In many respects this is much more modest than IMSLP – vocal music only, a much more limited selection, and not free to use. But because the scores at this site are Sibelius files rather than PDF scans, they offer the huge advantage of being transposable. For a very modest £1.40 per song, you can use this site to download any song from their collection, transposed into any key you like – invaluable if none of the published keys are quite right for you. Regular users might want to consider taking out a subscription (also very reasonably priced).
The Italian Madrigal Resource Center
Martin Morrell's site presents his own editions of madrigals by dozens of Italian Renaissance composers. The scores are free, though you need to go through a simple registration process to access them