Zarafa / Kopano:
Zarafa is no more, Kopana is here.
So far this is one of the most featured (open- source) groupware applications I have seen which is capable of replacing outlook because it has allmost simular functionality or better.
On the otherhand, it might be somewhat overkill if you only want to read your email online. But if you want a full featured groupware application supporting video conference etc this is the app for you !