There are a set of built-in extensions currently defined: FileDialog, LocalIO, NetworkIO, HTTP and LaunchApp. They each do what you expect, with NetworkIO being a generalization of HTTP. I'm busy implementing each of them between now and the end of next week. We aren't limited to these built-ins, however: any QScriptEngine extension plugin can be loaded, security willing, just by listing it in the metadata.desktop file. This means that widgets can be extended with arbitrary functionality.
The security framework is still in development, but all the hooks are there with some of the pieces in place now. Most importantly, we know exactly how we want to implement the missing bits. I'll be working on filling in the gaps as I can, and hopefully when Rob returns with hacking time again (or someone else steps up in the meantime) that will go a bit faster.
There are also a growing number of examples to augment the documentation and which are just begging for Techbase tutorials to be written around.
In an unrelated side note, Markey's blog entry on finding a balance in software configurability is really well written and makes several excellent points with great clarity. A must read if you haven't already seen it, in my humble opinion. :)