eM Client is a Windows only application that has been in development since 2006. Version 1.1 beta of eM Client already offers a pretty robust Outlook clone, with support for IMAP and POP email and CalDAV calendars. While it's not exactly an Outlook killer yet, eM Client's developers are working on a few killer features:
- Google Calendar and Contacts Sync
- ActiveSync support
- Universal translation tool
- Facebook integration
- IM integration
- Anti-virus integration
