Met Publications: hundreds of Metropolitan Museum of Art (New York) publications. In print items can be previewed through Google books and many of the out of print items are available for free as PDFs.
I think that it is awesome that many big institutions are now making their out of print titles freely available to the public. I think that they are realising that doing so costs them little and is more than made up for by the public interest it generates. Unlike our local museum which thinks if it puts anything up digitally people won't visit them, these museums and galleries know that actually making images and information available actually drives visitors to them.
And a word from their director (check out the amazing tapestry images)
Gothic Ivories: when doing object research e.g. costume, we often overlook statuary which is a shame because its 3d and can show us details that you'd miss looking at purely 2d art, although the reverse is also true.
Museum Boijmans is having a unique exhibition of early dutch art called The Road to Van Eyck. There's no mention yet but I am hoping that they publish a catalogue.