Microsoft's mail and collaboration plaftorm, which evolved from an X.400-like MTA into an SMTP-optimized behemoth with client-side MAPI, IMAP4, NNTP, POP3 and WebDAV support.
- Zarafa, another company doing a clone stack.
- OpenChange, an attempt at re-implementing the necessary protocols in a portable fashion, supplying MAPI libraries, Wireshark dissectors, the works.
- Brutus, a library and proxy connector to access Exchange servers.
- PostPath supplies a MAPI transport to emulate Exchange-like functionality on UNIX servers
- MS Exchange Blog
- The Exchange Project - Syncing Exchange to iCal and back
- Slipstick Systems - simply the best Exchange and Outlook site you'll ever find.
- Accessing an Exchange mailbox using WebDAV
- ASP.NET samples for WebDAV access.
Pseudo-Alternatives:
- OpenGroupware
- SUSE LINUX Openexchange
- exchange4linux - written in Python, of all things, but has a binary Outlook connector for Windows platforms.