Following a link from Wikipedia's page on IMSLP today, I discovered a similar site called Musopen, which makes available public domain sheet music as well as recordings (though a somewhat limited supply). Most of these are fairly high quality, recorded by a variety of individuals as well as the Skidmore College Orchestra and Wind Ensemble (the founder studied at Skidmore).
I have contacted the creator of the site to suggest that he get in touch with Feldmahler about merging their efforts. This would be especially beneficial since, with the exception of a few links to the Piano Society, it was very difficult for users of IMSLP to find public domain recordings of pieces whose scores they had access to.
I received a reply, in which the site's owner expressed interest in cooperation between the sites. He said that he opened the site in response to the closure of IMSLP, when it was uncertain of IMSLP's future.
The site had one feature that I found quite intriguing. It has a "bid" section, which raises money from users for recordings. There is a list of pieces that are certainly public domain (though it may be difficult to find PD editions), and prices. If enough donations are made for a particular piece or set of works, the owner will combine the donations and arrange with a professional performer or ensemble to record the piece and release the recording into the public domain.
Opinions?
Musopen Project
Moderator: kcleung