Clients
Microsoft Outlook - Email client from Microsoft, available as a part of the Microsoft Office suite (F)
Thunderbird - Formerly developed by the Mozilla foundation (F)
Providers
1and1 - Email hosting from full service web provider
atmail - Linux-based email (FT)
Gmail - Email service provided by Google for business (web-based) (FT)
Rackspace - Email and hosting service provided by rackspace