Mozilla Labs announced a new product called JetPack, which reminds me of the kinds of features you've seen in Adobe Air, Flock and Songbird, but the tool makes creating said features very simple.
Mozilla Labs Jetpack - Intro & Tutorial from Aza Raskin on Vimeo.
In the video, the developer mentioned two other Mozilla Labs projects, the first of which I hadn't seen before. It's called Bespin, and it's a cloud-based code editor. Right now, they are hosting the app for open source developers, but I'd love to be able to host an instance on my own server in the future.
Introducing Bespin from Dion Almaer on Vimeo.
Last but not least is what's basically an implementation of QuickSilver (the application launcher) in your Firefox browser, but instead of launching desktop apps, you're accessing web services, search and browser actions. The project is called Ubiquity, and it's cool (for people who like using the keyboard).
Ubiquity for Firefox from Aza Raskin on Vimeo.
Then they just get downright insane...
Aurora (Part 1) from Adaptive Path on Vimeo.
This video really takes the idea of web browsing away from just a bunch of flashing data and gives some good context. I can't say I would like to use that exact interface, but extra points for effort.
Or if you'd rather see something that's not so far in the future, this MIT student builds on some ideas that are already out there and improves them. It still has some of the "spatial history" ideas, and takes the idea of "pages" out of the browser, but if you ask me, it doesn't go far enough.
Firefox Concept Video from liyan chang on Vimeo.