Windows only: Evolution, the default office suite installed on most GNOME-based Linux systems, has a working port available for Windows systems. As its Linux fans know, Evolution has a serious focus on supporting and adapting to open standards: Full iCal support, IMAP access (I got a Gmail account working in minutes), integration with Pidgin's IM client, and support for GPG encryption. The big news for non-Outlook acolytes, however, is that Evolution can hook up to Exchange servers, though I haven't been able to test that personally. You also get contacts, memos, and tasks in the Evolution suite, and they're pretty robust in their own right. Evolution's Windows port is a free download for Windows systems; note that, while it installs, some have reported buggy operation in Vista.