A decade or so ago, ISI's EndNote bought out most of the competition, practically obtaining a monopoly on the reference manager business. In the early Library 2.0 boom, web-based products like Zotero and CSA's RefWorks became the norm. Thomson Reuters played catch up by introducing EndNote Web, and NoodleBib and other adware/freemium clones cropped up in what is now again a crowded marketplace.
Mendeley, recently purchased by Elsevier, has gained fame by offering social media integration and and sharing cababilities. It notably works on the old Questia model of selling itself directly to individual users, not institutions. ProQuest is also putting the finishing touches on RefWorks Flow, which features similar collaboration tools.
The way these newer products allow users to share articles with peers raises interesting questions about them potentially being used as a new "Napster for subscription journals," especially since they are now both owned by major publishers. See my comment for some more philosophical questions....