Web 2.0 Choice

Classifieds and Directories
1st Place: craigslist

I happen to agree with their ranking, as I use craigslist for nearly everything (I found a free cat, some books, and am looking for a chair). It is very user friendly, particularly for the college student. Everything from items for sale to job listings are organized by US city and subject and are easily searchable. The major drawback is that you have no guarantee as to whether a posting is legitimate or not, as anyone with an email account can create a listing, so just be wary! Contact the person directly and ask to see merchandise before you consider sending any payment. Deal in person if possible.
Craigslist is probably not very useful in a library setting, unless it were to serve as a tentative community resource when directing patrons, most likely for things like events in the area.