Zotero

If you're regularly using more than one computer in your research, Zotero's sync feature can keep your library up to date on all of them.  Zotero can store a copy of your library on the Zotero.org server and check it for updates whenever you open your library on a different computer.  All your computers must be running the same version of Zotero.

First, set up a (free, of course) Zotero.org user account. Then:

• Open Zotero preferences (via the gear menu) and select the Sync tab. 
• Enter your Zotero user name and password. 
• Check the "sync automatically" box.
• Check both boxes under File Syncing and choose Zotero storage for My Library. This will sync your PDF attachments as well as citations (more info).
• Click the green circular arrow button at the top right corner of the Zotero window.
• Zotero will upload your library to the server.

Repeat this configuration on each of your computers.  Any updates you make on one of your computers will be reflected on the others.  This even works to synchronize your library among Windows, Mac and Linux computers.

For more details and help troubleshooting sync problems, check the Zotero site.