DavMail + Thunderbird + the Thunderbird "Lightning" extenstion is pretty great. It gets you email, calendar, and active directory contact lookup functionality

DavMail + Thunderbird + the Thunderbird "Lightning" extenstion is pretty great. It gets you email, calendar, and active directory contact lookup functionality all without having to install Outlook. It's unfortunate that it requires Java though, because I try not to install Java on any fresh installs due to the security issues.