PIM by our friends at Microsoft. Client for MsExchange.
Marketshare leader, based on their classic model of giving it away. Also because some competitors dropped the ball (e.g. Mac-based NowSoftware failed to merge Now Contact and Now Up To Date, and add in Eudora).
Mitch Kapor started (Aug'01) supposedly working on a competitor (OSAF)...
I use (Jan'03) Mozilla for now (for EMail, it doesn't have other PIM features, though there's an add-on Calendar project).
Another option is http://www.bloomba.com/
For now there's Evolution, but it doesn't run on Windows or Mac.