Papers 2 was released in 2011 and offers full EndNote-style reference citation features. It allows users to access their libraries online and insert citations across different applications in documents, presentations and web browsers. Papers 2 offers a familiar user interface and a number of features for collecting, curating, merging and linking articles.
Papers 3 improves the way you find, organize, read and cite. A license enables users to access Papers 2 for Mac, Papers 3 for Mac, and Papers 1.5 for Windows. Three computers can use the tool at the same time. Paper 3 for Mac has a great workflow (according to the reviews) that you can use to organize personal libraries of documents. It also has Dropbox syncing across multiple devices and social media.
A Papers 3 license entitles you to use both Papers 3 for Mac and Papers 3 for Windows (as well as older versions). You can activate up to three computers at a time. Springer purchased Papers 3 in 2013 saying it "...helps you to collect and curate research material that you're passionate about; from citations to search...".
Sente 6 is an academic reference manager for MacOSX and Apple's iPad. Some of its key features are:
Mac-only software that can sync to multiple computers
format bibliographies in Word
capture citation data from some PDFs
share lists of references
As a free tool, Sente can search and retrieve references from sources such as PubMed, Web of Science, and sites supporting Z39.50 or SRU search protocols. Sente is often used to collect and manage pdf files, and is able to format bibliographies in APA, Chicago, MLA and others. Sente works with a number of word processors, including Microsoft Word, Pages (Apple), Mellel, Nisus Writer, OpenOffice.org and others. Sente for iPad, a free app for the iPad, provides access to Sente libraries on the iPad, and supports adding and editing references, reading and annotating pdfs. Sente libraries can be automatically synchronized across any number of Macs and iPads.