Mendeley is a citation management tool that, like many others, lets you collect & organize citations, & then easily insert them into documents & format bibliographies. But it also lets you drag & drop PDFs into your library & extracts the metadata to create a library!
You can highlight and annotate your PDFs and share them with others. Colleagues can then also annotate the same document, with each author displaying as a different color.
Use more than one computer? It's not a problem! Download Mendeley Desktop to as many computers as you want, then sync them with a click of your mouse. Need to use your library away from your computer? Mendeley Web gives you access to your citations no matter where you are.
Mendeley is social: create a profile, follow other researchers, create a network of people doing similar research. Discover the articles and journals that are being read in your field right now.
Create collections of papers. Make them public & share them with an RSS feed (great for class reading lists!).