Web apps are probably the most common because the barrier to entry is very low, but the field really seems to be more "software" rather than "only web apps". Xobni is desktop software, Loopt is mobile software, Justin.tv is (as I understand it) a mix of hardware and web streaming.
Interestingly, those also seem to be some of the higher profile projects.
Interestingly, those also seem to be some of the higher profile projects.