I stumbled on this diagram by Martin Fenner which documents the differences between various popular reference management packages. It was produced a year ago and is already slightly out of date, such as Mendeley now has an iPhone App and API - nevertheless - it gives a decent snapshot of some of these tools should you wonder what's the difference between CiteuLike and Zotero. It does not include Reference Manager, but does include Endnote which is made by the same company and is very similar.
