I've been working on several solutions with Horde, egroupware, OX, etc. Here is what I've found out.
Test bed config: SME test server is running php5/mysql5, outlook version 2000 with all patches.
Egroupware 1.2-105 will sync using the funambol outlook client for calendar/tasks/contacts but trying to share sometimes doesn't work
Using sun mozilla calendar addin to mozilla firefox will also sych by way of the ical standard but egroupware server always takes precidence. Egroupware has the possibility of becoming a good business intranet appliciation.
Open-Xchange on SME test server mysql4/php4
Using the outlook connector sold by OX.com, I was able to get it to working to sync. It was the only one that was always a live sync, not a timed or scheduled sync. But the web interface of OX is cumbersome and had lots of issues with sharing calendar events in Outlook 2000.
Horde/Kronolith - The 3.1.x versions are working well in the Web interface, never could get funambol outlook to sync to the horde framework. Found out its still in heavy development. I think a beta solution might arrive early next year.
Optional workaround, found workgroupshare. Runs on a windows 2k/XP pc, connector works on any version of outlook. It shares the PST files, does not exchange data like an MSExchange server would. Cost is reasonable for business, just not free. 30 day full trials are available.
Scalix - works well, but will not run on an SME server, must be on a dedicated box.