EarthPorn

I'm glad you asked! Devil's tower is "one of a kind" because the formation of such features are rare. It consists of phonolite porphyry which is an intrusive igneous rock (meaning it formed from cooling magma inside the Earth). Most of the time when you see columnar jointing it is associated with basaltic igneous rocks which are much darker because they contain higher levels of Fe and Mg (mafic) compared to the more silica rich intermediate igneous rocks like the one in question. There are similar geological features, though, including some in Iceland and Scotland (see giants causeway) and as for the hexagonal shape it turns out nature just likes to naturally form them because its easy.
I guess to summarize, there are not many similar formations to compare Devil's tower to, but those that do exist were formed from different magma (more mafic) and did not erode as gracefully as this formation.

Thanks for your response, yes that is one of four theories. Geologists have not come to a singular conclusion as far as I know, however. So the four competing formation theories are that it was a laccolith, a volcanic neck or plug - and more recently that it could have been a volcano that came in contact with ground water (Maar theory), or a stock. If you have further research I would like to read it!