First of all, I have to say that being a part of this here Web Challenge has been a TON of fun for me. As you can tell by scrolling down the page, I clearly like the excuse to ramble about web technologies. But even more fun has been reading all of the other players' blogs (okay, not all of them, obviously, do you know how many posts have been written so far? A whole hell of a lot!) and talking to people about web 2.0. And desperately trying to convince all of you nay-sayers that RSS really is worth it.
My favorite thing was definitely google docs. I didn't use it before, and now I use it all the time. I'm even trying to convince my department members that we should start using google docs for all sorts of crazy things, like meeting notes! keeping track of librarian-by-appointments! or maybe even... the schedule! I'm being a bit silly here, but honestly, using google docs has really changed how I approach the internet and working at the library in general. And the simple fact that I can access my google docs from any computer I'm using at work or home makes my life SO much easier (see also: why I love del.icio.us!). Honorable mention has to go to flickr and my old pals RSS feeds, because I use them day in and day out and would be very sad to give them up.
As for my least favorite thing, I'm not sure. I will say that the RSS feeds Things weren't concise enough... and I really don't think I'll end up using LibraryThing seriously. I keep a list of what I'm reading elsewhere, and since I don't have any friends on LibraryThing, I just don't see myself using it all that regularly. Also, Technorati. I wasn't a fan, although I don't think I'm alone.
I think the library can definitely get more involved with wikis, and del.icio.us (one of these days I'll stop talking about making a reference desk del.icio.us account for our branch and just do it), and google docs, just for the collaboration.
As for that loaded last question, what's web 3.0? I can't even imagine. I did a pretty rudimentary google search and read some stuff that frankly went over my head a little bit. A lot of the articles (like this one I saw in PC Magazine) mention something called the Semantic Web which seems to be something where intelligent computers can read the web sort of like we read the web, to find what we're looking for, effectively turning the web into one giant database. .... What? I think we may have to wait and see what happens with Web 3.0. But I'm pretty content to continue to spend some time using and exploring Web 2.0.