I thought I'd at least heard of pretty much every type of engine there is, but this one was new to me. It's probably what the infamous “air car” uses. I like it, it seems pretty simple, and it could run just as well on steam. Check it out here. He has diagrams of other types of engines that are also very interesting, you can see them here.
You know, the engines are the title discipline in engineering, maybe I should do a talk on the various types of engines? I imagine I wouldn't have much to say that isn't on these pages, but I'd still do it if there's interest.