Mendeley is a freemium scientific reference software created in 2008 and at version 1.7 as of Nov 2012. It can be used to store, categorize, cite, and share papers and runs on PCs, Macs, Linux machines, mobile devices, and online.
Free usage limits
- max. 1 GB data (Mendeley will automatically upload PDFs to the server if you link the desktop software with your user account) 
- max 3 users in a private groups (allows to share files in addition to the citation information)