The New York Public Library has released scans of 180,000 public domain items in their collections. “No permission required [for use]. No restrictions on use… go forth and re-use!” Nice, but actual access is currently rather clunky. Seemingly only via a gee-whizzy visualization tool that never stops its “Loading…”, so I couldn’t actually test what they have on offer.
Update: low-res only, access to hi-res versions is paid.