There's of course a lot of buzz about "Rich Internet Applications" floating around the Internets these days, and rightly so. The web is evolving and that means shrugging off the look and feel of static website with dynamic features to deliver user experiences that more closely resemble desktop applications.
There are a couple exciting things about this; first of all, it gives you the ability to build incredibly responsive, rich interfaces quickly and without a lot of custom code. Who wants to rewrite the same old dialog animations over and over again, after all? Also, as Ryan Carson points out, if you run an application of this calibre inside a SSB like Fluid, it can look and act just like the desktop app that it's replacing. Of particular interest to me is the fact that SproutCore is based on Merb, a lightweight Ruby web application framework, and is easily plugged into a Rails or Merb backend (but can be used with any other server side framework as well!)
Perhaps just as importantly, Sproutcore is being touted by Apple as the next big thing. In fact, it's been used as the basis for their new MobileMe service, and they've contributed a good sized portion of this work back to the framework itself. This is good news becuase not only because it gives Sproutcore a killer app out of the gate, but also because it throws the weight of a serious software giant behind it. If Apple is taking this seriously, then certainly other companies will follow in short order.
For more information on SproutCore, make sure to check out their hello world tutorial and documentation; it's a great way to get a feeling for exactly what's happening here (and how you might use it!) in under 10-15 minutes. SproutCore also hosts a number of impressive open source demos written using the framework that you can peruse.
For more on Objective-J, well, stay tuned...