Aliaksei Zholner's Youtube account features various small, clever papercraft engines that he's made over the years, but the latest one, measuring a mere 18 x 13 x 22 mm, is the daintiest, most lovely one yet, and well worth the long hiatus since Zholner's previous outing.
The engine is powered by compressed air delivered by a balloon via a throttled pipe (also made of paper!), and it audibly revs when it's going.
Zholner documented his build progress in Russian and has provided some downloadable templates for you to use in making your own.
His Youtube page contains this cryptic notation: "Специально для пользователей из стран бывшего СССР: все политические комментарии будут удалены, "особо настойчивые" комментаторы – забанены. Всем мир :)," which Google translates as "Especially for the user of the countries of the former USSR: all the political commentary will be removed, "very persistent" commentators – are banned. Peace for everyone :)" Apparently there is a political dimension to papercraft.