Rui CarmoThe Open Source Outlook killer. Or so a lot of people hope. It has recently (like Entourage) gained the ability to connect to Exchange servers via WebDAV, but calendar support via this method is flaky to say the least.
Resources:
- Accessing Exchange using Ximian Evolution/Connector - a nicely written step-by-step guide that is still mostly valid.
- Evolusync - to sync Nokia Series 40 (and soon Series 60) phones.