I just read an article from last week's Chronicle about CiteULike. CiteULike more or less works like del.icio.us, but it is for academics to tag what they are reading recently. It doesn't appear to have as much functionality as Zotero, which I wrote about last September. But--unlike the current version of Zotero--it is a shared resource. Others can browse your articles and you can browse theirs. What's more, the site draws from Emory's sfx service, and therefore can link you more or less directly to the full text of an article if we have access through our library's databases.
Based on what each tool can currently do, CiteULike is a much more useful tool for research. It allows you to watch what others are reading and to take your references with you (because they're all online). Zotero has a better interface, in my opinion, and is more full featured. But until the next big release, it is tied to your local browser. And that's more or less what EndNote is.
I don't know that I'm going to start using CiteULike regularly. But that's got more to do with my not having internet access of my laptop than anything else.
(Cross-posted with the Emory English blog, to reach multiple audiences.)