I am a huge fan of Zotero, and I am thrilled with the latest upgrade to version 3.0.
The previous versions of Zotero only worked with Firefox. While this was great, the latest version is a standalone version that has integration with other browsers such as Google Chrome and Safari. Since I am a Google Chrome user, this change has made my life much easier. Now, I do not have to switch between browsers while working on research. The new version of Zotero also works on Mac, PC, and Linux.
Other meaningful changes include: duplicate detection and merging, ability to drag collections into group libraries, and improvements to word processing integration.
If you use Zotero, I would recommend the upgrade.