This project has moved to <http://christianserving.org/project/truepreview>. TruePreview is a plugin for Apple's Mail.app that allows you to truly preview messages.
Ginko - A mail and news client for OpenStep platforms. Initial targets are Mac OS X and GNUstep. Ginko will provide a unified view on mail and news while being optimized for 'disconnected' work.
A plugin for the MacOS X Mail application that adds the ability to fetch mail stored on a httpmail (Hotmail) server