What progressive disclosure techniques where the information and options given starts out small and lets the user decide what they want expanded on can I use in a web app?
Here are a couple of excellent articles on responsive disclosure:
You could also consider looking into the responsive enabling pattern.
You can do the following:
I also like the approach of waiting for the user to express interest in an object, usually detectable by hovering over it with the mouse pointer, and then (after a slight delay perhaps) automatically reveal more options on that object.
For information only (not options)
You can make something like Google Wonder wheel where you start out with a small amount of info and as you click you get more options.
This is not for everything and is limited to browsing large data sets.
Another option for large data sets is the new silverlight pivotviewer (click launch pivotviewer)
These are both good for veiwing large data sets and have nothing to do with functionality