A good reference management software can make the life of a researcher much easier. There are many different software available, including various free or open-source software applications. My favorite tool is Zotero, which is a Firefox extension for the collection, managing, citation and sharing of your research sources. It is very powerful and has become indispensable in my research work.
A very nice feature is the option to store your citations online and share them with others. Particularly the option to create shared libraries (groups) makes it an ideal tool to maintain a shared library of references for ongoing research. You can make those groups public (anyone can view your group online and join the group instantly), public but with closed membership (anyone can view your group online, but members must apply or be invited), or private (only visible for invited members). See for more detailed information here.
Currently the focus is on sharing of references between Zotero users. It offers less functionality for users on-line. The web interface only presents a list of references and sub-libraries (folders), but there is no option to sort references or search on-line within libraries (a very serious omission in my point of view, but it seems to be on the to-do list of the developers), or social functions like the option to leave comments. I am sure new options will be added, but for now, if you are looking for an alternative on-line reference manager (e.g., for when you travel), you may be better of with e.g., Mendeley, Connotea or CiteUlike. Note that Mendeley also offers a desktop client. Although not open-source, it is free and has some interesting features, especially the option to synchronize its database with a Zotero database.
For an example of a Zotero group library, see the Forestalis group library. Forestalis is a cooperation between six professionals (of which I am one) in the area of natural resource management. Our website present news and articles on subjects that concern us, engage us, or amuse us. The Forestalis group library presents an ideal tool to keep a record of those articles and news items. Moreover, it allows us to share the sources we would like to recommend to other people with similar interests, albeit for now options for these users are limited to simple browsing of the reference list.