Lightweight System to replace TeamSite

System

I swear that the decision to replace TeamSite was well under way before I joined this project as users, developers and adminsitrators have always been unhappy with the tool for a variety of reasons. In general the existing implementation is pretty bad (thanks XUMA) and it is just too complex to adapt to emerging requirements. I have been tasked with identifying a lightweight tool to replace TeamSite, but with some additional features. My first idea was a wiki but I don't have any experience implementing them. Some of the features required of the toolset include:

Chat capability
Messaging/threads
Content administration/file versioning
Survey data capture and reporting
Intuitive
Scalable
Preferably integrates with .NET, alternatively J2EE
Open/component architecture (this system may need to be swapped out).
I would prefer that the back end be a database.

For budget reasons it probably has to run on Windows or Linux. I am open to Open Source. Any suggestions for such a solution would be greatly appreciated.