Mendeley

"Mendeley is a free reference manager and academic social network. Make your own fully-searchable library in seconds, cite as you write, and read and annotate your PDFs on any device."

• Create bibliographies of various source types including books, articles, websites, videos, and more
• Create a fully searchable library of full-text documents
• Highlight and annotate documents, and share your notes with other researchers
• Organize sources using folders, groups, tags, and filters
• Collaborate with other scholars, students, and researchers through an online network
• Find additional relevant resources based on what you are reading

Some of the features available with a free Mendeley account include:

• 2GB of storage (additional storage is available for purchase)
• Full integration between Mendeley platforms
• Access to a free and open database
• Paper recommendations
• Group research collections 
• Citation plugins for word processing software
• Online/Desktop/Mobile Platforms
• Social networking with other researchers