What's one of the most amazing, cool, web-based apps that no-one talks about?

Trac.

It's not flashy, or Ajaxy. But this packs so much useful functionality. Every time I show this to a developer, they drool.

Right now, if I was building a wiki-like application product for use in enterprises, or something to compete with Basecamp, I'd seriously consider starting here rather than with the usual Rails or Turbogears etc. It feels so well thought out that I bet the code is nice. And it's extensible.