Bibus is one of the more functional open source bibliographic managers. Based on an SQL backend, it will run in Linux, Mac, and Windows OSes.
Pros: The interface is pretty intuitive, and it is easy to create style files for output in specific journal formats. Bibus also includes a feature akin to "Cite While You Write," compatible with both Microsoft Word and OpenOffice.org Writer. Manual input of references is pretty easy.
Cons: The Bibus development team is quite small, and it can be a looooong time between updates. There is no support for character formatting in the database either (italics, underlining, etc.), which is quite annoying if your references have scientific names in them. Additionally, it's not entirely straightforward (although certainly possible) to import bibliographic information from journal websites. In the current version of Ubuntu Linux, lots of folks are having trouble getting it to link in with OpenOffice.org. This is the main reason I abandoned Bibus for the package discussed in my next post. . .
The Bottom Line: Bibus is pretty functional, but has its quirks. It's a good choice for Windows and some Linux users, but requires a little effort sometimes.