Some of the participating institutions include the British Library, the Louvre, the Biblioteca Nazionale of Florence, National Library of Spain, the Federal Archives of Germany and on and on an on. I assume that as more and more collections are digitised they will be added to the database. The site is incredibly useful as a sort of 'one stop shop' (if such a thing could ever exist in research). I have already found a couple of digital databases useful to me that I didn't know existed, such as the Ministero per i beni e le Attivita' Culturali, which holds a lot of images of Italian villas and gardens. It also makes it much easier to search collections that have strangely organised and inefficient websites where you seem to continuously click through to another page telling you about the catalogue and never reaching the search function (I'm looking at you Italian libraries!) It also helps with searching collections where language is a barrier. You can even create an account and save your searches and records.
I am currently writing about the Boboli gardens in Florence and unearthed some old postcards of the gardens. A teeny example of what is available.
View of the Pitti Palace and Boboli Gardens, 1905. Courtesy of ICCD.
The Avenue of the Cypresses, Boboli Gardens Florence, 1903. Courtesy of ICCD.