Discovering Web 2.0 Tools

I had a look at Craigslist, as I always hear about it on various American movies and TV. It seems to focus on local areas, which is a bonus, and has lots of forums, advertisements for jobs, sale items, etc. On first galnce it looks a little chaotic, but with some navigation I could see it being used as a tool within libraries for communication and other uses. There might, however, be better (and more secure) tools out there, with more specific requirements for libraries.