Zotero 2.0 beta was recently released. I was using Zotero 1.0, and while it was handy, it was too difficult for me to manage my citations across the multiple computers I use. 2.0 fixes that problem with a handy sync feature that allows you to sync Zotero to multiple computers/browsers.
Zotero also allows scholars to open their research collections to others, including making them publicly available. I can follow people working in my field and have access to their source materials quickly and easily. I imagine this could enhance conversations and collaborations about ideas and research.
You can annotate your citations in Zotero. When searching common databases like JSTOR, Zotero will save the full text pdf of articles, making the full pdf searchable in the software interface.