Ximian Evolution[tm] is the premier personal and workgroup information
management solution for UNIX. The software seamlessly integrates email,
calendaring, meeting scheduling, contact management and online task lists in
one powerful, fast, and easy to use application. Ximian Evolution also
delivers a comprehensive set of features that help you keep your work
organized and your projects on track. Automatic email indexing keeps your
information at your fingertips. The unique Ximian vFolders[tm] are virtual
folders you use to create and save powerful contextual views of your email
messages. 

WWW: http://www.ximian.com/products/ximian_evolution/