The International Music Score Library Project, now also known as the Petrucci Music Library, was started in February of 2006 as a repository for public domain sheet music. It quickly attracted attention and gathered a sizeable collection of music, using a wiki format that allows people to easily become contributors.
The IMSLP is hosted in Canada, and operates under the copyright rules of that country, which currently grant protection for the life of the author plus 50 years. Thus, there are many works which are hosted there which are not considered public domain in the USA. (However, there are a few instances in which the reverse is true, for some authors who were particularly precocious and/or long lived.)
When Art Song Central was begun in 2007, IMSLP had been temporarily shut down due to a legal battle over copyright issues. However, it has been back in business for a while now, with what appears to be a more robust legal grounding, and has continued to grow at an astonishing pace.
The collection is already so large that it would be impractical for ASC to try and catalog all of its vocal holdings, but I will try and note worthwhile additions as I discover them. This will be done in a similar fashion to the previously referenced complete songs of Schubert and Schumann’s Dichterliebe, so that people will be more likely to find a free resource when they search for a song here.
One thing to note: Although this is likely to improve over time, the scores at IMSLP are still somewhat variable in quality, with some contributions being not quite useable for performance. I’ll try to only link to the good ones. 🙂