So I've looked at least briefly at:
Haven't looked at:
Prolly won't look at, cause they involve flash:
But I dunno. You can do lots of pretty cool stuff with Flash. Even thought there might be a possibility of this stuff running on a PocketPC or something, I guess PDAs are soon to be dead, so the point there is kinda moot. Actually I took a quick peek at Zimbra late last year and realized they didn't really have a story for running outside of a JSP/Servlet environment. All of their component .js files got pulled in via a .jsp. Um. Ick. It's the new millenium guys.
Thoughts on the ones I've looked at:
dojo has a lot more stuff than yui and mochikit. dojo's documentation is quite thin, compared to the other two though. Besides just doc, or maybe just because of the doc, the other two seem more professional. But you know, there's no reason you couldn't use all of them at the same time. You would hope. In fact, I think I saw in the mochikit stuff some dojo-sniffing and -enablement.
Taking a look at MochiKit again right now. It's pretty slick.