Mendeley

Automatic Citation Capture: Automatic extraction of document details (authors, title, journal, etc.) from academic papers into a library database, which saves you a lot of manual typing!  As more people use Mendeley, the quality of the data extraction improves.
Source Management & Searching: Super-efficient management of your papers. "Live" full-text search across all your papers - the results start to appear as you type! Mendeley Desktop also lets you filter your library by authors, journals, or keywords. You can also use document collections, notes and tags to organize your knowledge, and export the document details in different citation styles.
Synch Your Files: Sharing and synchronisation of your library (or parts of it) with selected colleagues. This is perfect for jointly managing all the papers in your group or lab.  
OCR (image to text conversion): So you can full-text search all your scanned PDFs.

Online Back-Up: Mendeley serves as an online back-up of your library: Store your documents in your account and access them from anywhere through your browser.

Statistics: Detailed statistics are available. You can upload your own publications to your research profiles, then track the evolution of your readership. How often are your papers downloaded or read? From which academic disciplines and geographic regions? Additionally, there are detailed statistics for each academic discipline and research topic. Who are the up-and-coming authors in your discipline? What are the most widely read papers on a specific subject?

Networking with Colleagues: A research network that allows you to keep track of your colleagues' publications, conference participations, awards, etc., and helps you discover people with research interests similar to yours.