Ben Smither has created what must be the minimal Segway-type vehicle:
More info here.
Trevor Blackwell has also made some cool devices – I’ve ridden his home-made Segway variant and it’s very good, in some ways rather better than a standard Segway. The steering is more intuitive, for example.
He did admit that a fair amount of the cost and complexity of a commercial Segway comes from the numerous safety features they build in, which he didn’t have to the same degree. He told me this after I’d tried it, of course…