CiteULike is the grandfather of all social reference management tools. First released in 2004, CiteULike pre-dates the development of both Zotero and Mendeley by a number of years. The social bookmarking tool for academics is an easy way to save references you discover online without leaving your browser. While less advanced in the processing part of reference management – unlike Mendeley and Zotero, CiteULike doesn't have a "Cite While You Write"-feature – it does offer numerous ways to share your collected references in groups and forums. In a way, CiteULike is midway between the more traditional reference managers, such as EndNote, and the social reference managers Mendeley and Zotero. If you want to have the more powerful reference managing and processing powers of EndNote, but also want to have the social component, CiteULike is a useful tool to achieve that, serving as a social layer on top of EndNote.
Not just a reference manager
CiteULike is not just a reference manager. Its social features allow you to use the service as a collaboration tool to crowd source papers that are linked to a particular project or subject. You can use it as a discovery tool to recently published papers in your field and see what is said about them using Citegeist. Or you can use it to discover who is reading similar articles and so is presumably doing similar research and might be worth reaching out to.
Depending on the reference managing tools you already use and on what you want it to do for you; CiteULike could be a useful service to check out. If you want to learn more about CiteULike, check out the In Depth page.