Hi Jane,
Ah..this is right up my alley. I know others will speak up about other projects, but, the Walters Art Museum is a pet project that I am involved with. They released over 20,000 CC licensed images, added the statement to their website, and we uploaded them to WM Commons. We're in the process of updating the case study I wrote a while back. You can find a link to it, and get some basic info, here:
http://openglam.org/2013/01/22/walters-art-museum-a-case-study-in-sharing/
We're in the process of working with the Walters to license their metadata CC0. We're SUPER close to getting the internal buy-in. I'm hoping we have that set in stone and the license will be changed (right now it's CC BY) in order for... the Walters hackathon, sponsored by Wikimedia DC and emphasizing creative reuse.
A second pet project that I am not as deeply involved in, but, am deeply passionate about, is the work the Cooper-Hewitt has done in NYC. Their metadata is CC0, they have a great knowledge-sourcing project with their online colleges database where you can easily submit changes through Zendesk, and tag any images or media related to the objects that you have on Flickr, INstagram, etc and CH will list it on their collection page. They also support using their collections on Wikipedia and provide you with a citation to do so on each collections page. It will only get better once the institution re-opens again.
I also facilitate projects at the Smithsonian's Archives of American art regarding a content donation - however, further content donations by mass upload (i.e. not just a person going and taking a PD image off their website and uploading it to Flickr or Commons) hasn't taken place recently, that I am familiar with.
Feel free to contact me with any direct questions!
Sarah