MicrosoftCorporation's GroupWare client. They describe it as "PersonalInformationManager and communication solution." 
http://www.microsoft.com/office/outlook/default.asp
Talks to MicrosoftExchange, IMAP, POP3.
With Bynari's InsightConnector, can do shared calendaring against Cyrus IMAP.
There are lots of addons for it.
Famous for its lax security which lets dozens of internet worms propogate.
