At its core dnstwist creates permutations of domain names and checks to see if they are registered. It also does a ton of other cool things like GeoIP mapping to resolved IP addresses.
As I am sure there are people who would like to use dnstwist but don't have Python installed, I have (after a weekend's re-familiarisation with Google App Engine) created DNS Twister!
It may look rough as **** but it works, presenting the core dnstwist functionality via a web application.
The source code is on GitHub of course.