Bookends is a Mac only citation database that allows you to:
Compatible word processors:
Online Search - Bookends allows you to search and upload from many online sources including PubMed and Web of Science.
Attachments - You can attach a PDF, webpage, or anything you like to the citation in Bookends.
Tag Clouds - Tag clouds allow you to see terms associated with the article at a glance. The more frequent the term the bigger and brighter the it is.
Reference Cross-Linking - Linking your references allows you to see relationships easily and allows you to make comments, notes about a particular quote, or type something that may be helpful later.
Sync - You can synchronize your database with other members of your group or between devices.
Bookend's provides video tutorials for some of these functions.