Open source. Run by the Mozilla foundation and was part of the original Netscape (the first browser) suite of software before they separated the browser from the email into two applications, this and Firefox. Netscape is still available Easy to use and reliable, includes many community developed plug ins including calendar which syncs with other calendars such as Google calendars.
Outlook
Microsoft product with many variations. Outlook Express comes with Windows while Outlook 200, 2003, 2007 etc are part of Microsoft Office. Due to being bundled with these applications it is used by more office workers than any other email program.