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!).
Mendeley's free version has some limitations on space (2 GB web storage, 500 MB each for personal shared storage) and users (3) and shared collections (1), but you can purchase premium plans for more web & personal storage space & more users & shared groups.
At Mendeley Feedback, you can suggest changes to the program, see how many votes your idea has, & if Mendeley is going to use it.
If you need more help with Mendeley