First of all, it's a good idea to move users away from exchange and outlook and their "standards" to something that's entirely based on true open standards. However, if your users are particularly cantankerous, below are some options.

• Apparently there is "MAPI the API" and "MAPI the protocol".
• MAPI is what gives Outlook its souped up features, relative to your typical SMTP+IMAP server.
• MAPI the API is apparently poorly documented
• MAPI the protocol appears to be undocumented

• otlkcon - Affiliated with Kroupware. Probably MAPI the API, not MAPI the protocol
• Lotus Notes
• Ximian
  • Connector is a commercial plugin for the free Evolution mail program. The plugin enables MAPI I believe. Rumor has it Connector doesn't speak the MAPI protocol, but seems like it because it parses microsoft's HTML (WEBDAV) interface to get comparable functionality. Further, evo's "connector" for exchange reportedly also uses LDAP on the exchange server. This Slashdot article indicates connector is now GPL'd, and will be included with future versions of Evolution. Sadly, even though Connector is free now, OWA (Outlook Web Access, which connector uses instead of MAPI) still costs money beyond a default exchange install.
• samsung
• contact - the server can do MAPI, so I'm guessing the client can too.
• More about contact from newsforge, with some comments at the end that touch on other products as well.
• These guys claim that they have more MAPI experience than anyone outside of microsoft - possibly even more than at microsoft
• They also claim to have made more of a dent of the Fortune 500 than any other Exchange replacement
• centrinity
  • firstclass - probably MAPI, probably have a client

• openchange
• OSER - a bunch of FLOSS stuff, but then there's qmail... :(
• SuSe is apparently planning an " Openexchange" to be released in mid-November of 2002.
• Lotus Domino - need Lotus Domino Designer Release 6 for MAPI
• SCO
• formerly volution - only supported on Caldera OSes?
• SCO Office
• Basically just don't buy their stuff. These are the guys who are trying to kill linux
• samsung
• contact
• Nov 7, 2002: Reportedly doesn't work with the Ximian Connector Plugin, but they plan to talk to Ximian about making a version that does. Jul 9, 2003: Reportedly has full Outlook/MAPI compat
• Communigate Pro by Stalker Software reportedly works, drop-in, with Outlook or Evolution with the Connector plugin. Jul 9, 2003: Partial MAPI support?
• billworkgroup - server code is opensource, mapi plugin is proprietary.
• centrinity
  • firstclass - probably MAPI

• fetchexc - a java program that can, in the spirit of fetchmail, retrieve e-mail from exchange via WebDAV.
• sherpath: opensource, web thing
• easygate: gets MAPI API (outlook compat) through exchange4linux (full compat with outlook?)
• Oracle Collaboration Suite - formerly Steltor CorporateTime
• Kroupware (KDE). Not MAPI, but see otlkcon above
• phpgroupware reportedly includes webmail, a calendar, and a number of other interesting things.
• MailStudio - works with outlook, includes a web interface, doesn't do MAPI (?), but it does do iCal (!) and apparently outlook supports iCal
• citadel is reportedly headed in the direction of outlook compatibility
• netscape calendar Cross platform they say. Opensourced, rumor has it. Is it ICAL?
• Amphora has commercial and gpl versions. Probably just offers comparable functionaity to MAPI
• Suse Linux e-mail server - I doubt this interoperates with outlook, but it does include e-mail and calendaring
• OSAFoundation is apparently working on something kind of like exchange, that doesn't require a server. Huh? Anyway, it's something to keep an eye on.
• Chronoss - just a web calendar

• OpenChange - to be an opensource exchange clone.
• netline open xchange
  • Affiliated with SuSE
  • Opensource
• Bynari
has a variety of products that may help linux users talk to exchange and/or outlook users talk to linux servers. However, Bynari's product descriptions are so steeped in marketing-speak that I can't always tell how the products help. I wish they'd just say "does IMAP, SMTP, MAPI, client side or server side". Instead they have a lot of inspecific stuff about how great their products are.
• This article is an example. Clearly they think they have something good here, and it's pro-linux, but just exactly what does the product really do?
• This one's clear though: It's an exchange client for linux and solaris. I suspect that it does SMTP and IMAP, and then there's an Outlook plugin for some proprietary protocol only spoken by Bynari for MAPI like functionality.
• Jul 9, 2003: Reportedly has partial Outlook/MAPI support
• May 11, 2004: Rumor has it they're no longer offering an exchange client
• OSAF
• CNET on Chandler - maybe MAPI, maybe not. Who knows?
• OSAF on Chandler

• Here's a gpg plugin for outlook
• quotefix