Mac users have several good choices available when it comes to bibliography-management applications. Bookends is among the best, and its trial version certainly merits a look from anyone who spends much time working with large numbers of academic and technical citations. This app makes it easy to search for, download, and organize reference materials, as well as quickly create tightly formatted bibliographies in a variety of word processors--including excellent integration with Mellel and Word 2008.
You can search across and save documents from any standard source, from PubMed, JSTOR, and the Library of Congress to Amazon, Google Scholar, and even any individual institutions to which you have access, and Bookends can even import journal articles into existing bibliographies. The interface is not particularly attractive but it's hard-working and fairly customizable, with many different options for ordering, labeling, and groups. Bookends' styled virtual note cards can be a big help when you need to annotate your references, and its new features like tag clouds and metatypes can be invaluable in organizing and navigating your documents.
With a built-in HTTP server and good third-party app and database integration, Bookends is also a good choice for collaboration, even with users using other reference-management applications. There's a lot of competition in this category, and Bookends has only gotten better in recent versions.