Convea – Open Source Business Application Platform: This is a pretty stunning piece of work. It’s super-Outlook in a browser. IE 5.5+ only. Try the demo.
If you’re ever used Oddpost, you’ll be just as impressed with Convea. Using astounding client-side programming, they have utterly and completely replictaed a client-side groupware tool in a Web app. Email, file manager, discussion groups, contact management, etc. — it’s all there, and you’d be hard-pressed to distinguish it from an installed application.
Oh, and it’s open-source.