Knowledge Management Systems

Can anyone suggest any suitable alternatives to Lotus Notes as an Enterprise Knowledge management system.

We are currently utilising MS Exchange 5.5, but feel that the GUI for Lotus Notes is far superior. if we can find bolt on software to Exchange that will provide this functionality, then this would be ideal.

I am putting this information together as part of an evaluation of other products on the market as Lotus Notes will become part of this report. However alternatives are very important, and I would like the knowledge of those of you out there with other systems or indeed, have used other systems and would recommend Lotus Notes.