IM'ing has long been a favorite and has to a large extent replaced e-mailing. This allows for chatter about all sorts of things, but also getting quick replies from friends and colleagues to short questions - stuff that you would have had to use a phone or an encyclopedia for in the analog past.
My favorite for questions is the internet forum - there is one for virtually any topic. Forums have to a large extent replaced usenet groups and there are always lots of very competent people to reach there! My favorite tech forum is ubuntuforums.org; you can get answers to all sorts of questions in no time!
On Mixx and Digg you can search for web pages in a way that Google doesn't do: you find sites people have bothered to tell others about. If you find some page tagged with your search keyword, the chance of hitting a good site for your purpose is good!
Then you have the more traditional information sites, like Wikipedia: always at hand and highly accurate (of course, for "real" work, you'll have to check again, but Wikipedia is usually really, really good).
Last, you have online apps, like Google Docs - do calculations in online spreadsheets, write reports and make presentations: all of this goes into the Web 2.0 stuff - the best part of Google Docs is the collaboration tools it brings, allowing efficient group learning and discussions.