Ximian Evolution™ is a "collaboration tool" and "groupware suite" written by Ximian, Inc. It's part of Ximian GNOME distribution, and works on *NIXes on which GNOME runs.
Practically, it includes
and other applications for such tasks. It also has a "summary" screen for news and such.
The application itself is under GPL, but Ximian Connector (module used to connect to Microsoft Exchange system) is payware.
Evolution is a gigantic application, similar to Microsoft Outlook (even including the obviously imitated user interface...), but it is built on open standards. E-mail is handled with SMTP/IMAP/POP3, calendars with vCalendar, and contacts with vCard and LDAP. It can also import some proprietary mail folder formats (Netscape, Eudora, and Outlook Express) while it supports normal UNIX mailboxes, too (it uses its own folder format, though).
It also interfaces with some PDAs (Palms, most notably, via gnome-pilot). I think it's a pretty neat replacement for jpilot - the GNOME sync thing works pretty nicely.
HTML mail is also supported to accommodate the needs of corporate users (read: PHBs).