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…
Oh, and you should see what he’s been up to at Anybots. Have a look at this and this.