Extending waves could be done with gadgets and bots. Bots were essentially XMPP clients that could observe the wave and submit changes to it [1]. Gadgets had other utilities, like introducing features to a wave document. I can't remember any particularly interesting ones, but some simple ones would be dice rollers and polls.

[1] I played with a clojure REPL bot at one point, neat concept and I liked the idea for a collaborative way to teach someone how to program or pair program. I toyed for a time with exposing a common lisp REPL (I didn't get far) to create a collaborative worksheet-style interface. Since the underlying technology was language agnostic I thought it would have been neat as a way to integrate a number of different interactive languages.