I’ve written about Zotero previously. I admit to liking it and if fits my needs. Still, I was intrigued by Mendeley and can see that it is very useful.
I located a Wikipedia article “Comparison of Reference Management software” that had several useful tables comparing a number of products including the three featured in Thing 14. I found this helpful. (http://en.wikipedia.org/wiki/Comparison_of_reference_management_software)
Perhaps more helpful though was this 2011 item from UCSF libraries which compared Zotero, Mendeley and Papers http://www.library.ucsf.edu/help/citemgmt/more.
The jury is still out for me on what I think of CiteUlike. It is indeed a kind of social bookmarking and that still seems rather foreign to me. That said, even if I understood and appreciated social bookmarking more than I do at this point, the downside that Isla mentioned in her post (there is no cite while you write functionality) would be enough to discourage me from using it.