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 more focussed selection, and not free to use. But because the scores at this site are Sibelius files rather than scans of printed scores, they offer the huge advantage of being transposable. For around $2.50 per song, you can purchase access to digital and PDF versions of any song from their collection, with the option to transpose into any key you like – invaluable if none of the published keys are quite right for you. Unfortunately Schubertline no longer offer an annual subscription, but purchases of individual scores/songs are still good value.
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