What is a useless machine? Something that has been developed to do absolutely nothing of use? Perhaps, however the following example by Instructables user "rsucgang" has created a version of such a machine which is simple yet somewhat amusing. It's a device that once turned on - turns itself off. And that's it - completely useless, however there's some construction and the results will amuse the younger ones - for example:
If you have the required parts but not the enclosure, one could be constructed from LEGO or even heavy cardboard, just for some quick fun. Either way, visit the Instructable page to make your own.