Zimbra appears to be an e-mail and calendar package with a gorgeous web-based client, and a server with a web services interface. Existing clients are supported for the standard mail features. There's some Microsoft Exchange interoperability here, too, though I can't tell if I'll be able to use Zimbra at work right away (not?). The Flash demo of the web client is impressive.

Zimbra is making their core technology open source, and using an open source development community to build and polish the product. It sounds like this is mostly for the interoperability pieces, a great idea.

Speaking of office apps in a browser: Writely, The Web Word Processor.