There isn't a single laundromat that does not have a bulletin board where people looking for a job, offering positions or searching a for a lost dog, post their information. You have seen those single sheets of paper with tabs at the bottom, usually listing a phone number or a address, where interested parties can make contact with the posters.
A similar concept is the "information kiosk", designated places, frequently found at schools and universities where people post information they believe is of interest for others.
Reddit, Digg and StumleUpon are, in a way, the electronic version of these information kiosks. They are a common place in the Internet where users can post interesting sites they have found that, they believe, should be viewed by others. It is actually a very nifty concept. Let's say for example, that I fing either an extremely funny site, the web site of a foundation that offers scholarships for library students, or other information that I deem of general interest. All I have to do is, after getting an account, tag or bookmark the website of my choice. Any other user of these applications would be able to see it from there.
The websites tagged using these applications are ranked based on their popularity. It is actually a very basic system. I tag a website because I find it interesting and worth viewing by others. Other people would likely find it either from inside or outside the program and tag it as well. if the website is popular, it will be viewed by many people and tagged accordingly by those who have the ability to do so, therefore entering in a vicious circle where interesting sites are viewed, tagged, viewed and tagged, ad infinitum.
I can see the utility of these tools for a library concerning news on the web. someone who is looking for the most relevant bit of news would look in one of these sites for the most popular web sites containing news. This way, the user would find out, from web users who have Digg, for example, what is the most talked about bit of news floating on the web. Librarians can use Digg accounts to tag popular web sites according to subject, gathering the most popular (dug) sites within the subject, ad allow users to browse them. It is a rather crude form of web information organization, yes, but a form of organization nonetheless.