Here is a Berkeley site with nice tutorials, in depth descriptions of each program, FAQs and more. Discussed are EndNote, RefWorks, QUOSA, and Reference Manager.
There is also this detailed side by side comparison of several programs including RefWorks, Biblioscape, Bookends, Library Master, ProCite, EndNote, Reference Manager, and Papyrus:
Tufts also has a nice page comparing Endnote, RefWorks, and Reference Manager.
For an overview at a glance here is everything about the main features of the programs put together in one big table (from a