Take a look at Warren's blog post.

Mendeley:

Digital libraries are collections of documents stored in digital formats as apposed to print, microform or other media.   These collections are made available by computers.  Most libraries now contain both digital and non-digital collections.   

Large scale digitization projects are underway at Google, the Million Book Project, and Internet Archive.   Google has an interesting webpage that talks about their vision for "The Future of Google Books".   Just as libraries have ventured into audio and video collections, so have digital libraries such as the Internet Archive.   Making the documents available is but one step in the right direction.  In today's Web 2.0 world access to digital media is not enough.  Collaboration is key to today's savvy user.

Mendeley is an example of what is termed Social Bibliography Tools.  These tools allow the user to organize, tag, annotate, and SHARE papers and articles with colleagues.  A set of videos to help you better utilize the tool can be found here: