If you use BibTeX for managing paper references (and after all, who doesn't) and you use a Mac, then you should know about BibDesk, an open source tool for managing BibTeX bibliography files.
BibDesk is at a great intersection: open source tool used by people who, for the most part, know how to program. There are frequent updates and the set of features is impressive. For example, I love the ability to not only manage the bibliographic data for an article, but the PDF as well, if I happen to have it. It turns my BibTeX file into a reference library.
The keywords are great and the preview, which shows what the entry will look like once TeX get a hold of it is very nice. Sharing of BibTeX data via Bonjour is also cool. Recommended.