Zimbra is an open source collaboration tool. It contains a very rich, DHTML email and calendar application. I’ve noticed that it’s a little sluggish on my browser, not quite as snappy as a desktop application. But it’s far better than what most ISPs offer today.
I especially liked the way you could plug into their smart tag system, allowing you to turn text into hyperlinks that do various things. In the demo, they show some sort of order number that you could click on to approve. I could imagine such a thing being useful in corporate realms where you could turn Remedy ticket numbers or Bugzilla bug numbers into hyperlinks.
It is produced by an actual company (they have a job listing posted if you’re looking), so I’m curious to know how they make their money. It appears from their Why Open Source page that they’ll make it on support and warranty contracts. We’ll see.
In the meantime, if you want to play around with it they have a hosted demo. The Flash demo isn’t bad either.
I particularly like Zimbra’s vertical left-hand icon-menu … it’s very “apple mac” and it’d look great on the new yahoo mail beta so peeps can get to their notepad/addresses/calendar/etc.