What are your favorite digital libraries, archives, and collections?

What are some of your favorite digital libraries, archives, and collections? I don't mean what has your favorite content, but what is your favorite user interface? Which is easiest, simplest, most clear, most fun, prettiest, cleanest, most accessible, or simply best tied into your everyday workflow? What do you like about it? What do you wish were different?

(You might notice that this entry is crossposted from its new location, deborah.dreamwidth.org. I'm going to be cross posting from the new location to gnomicutterance for the foreseeable future, and everybody with a livejournal account can still comment over there using OpenID; your livejournal account is your OpenID account. Anonymous commenting is also turned on, as always. Gnomicutterance.livejournal.com is still going to be a functional address which mirrors all those posts, but I find Dreamwidth's commitment to accessibility and usability makes it more attractive than Livejournal.)