There doesn't appear to be any other service that provides this functionality. You could just sign up and create your Avatar and then revoke the Application access in http://twitter.com/settings/connections
You can also create these types of images using PhotoShop - a simple Google search will provide you with lots of sites with tutorials on how to do this.
According to this article on CrunchBase the images are created using pure CSS & HTML.
Eightbit.me will be a service to help
people create 8-bit avatars. The first
idea was to use images with very few
selections and to use pure HTML / JS
to create the characters.
Inspired by an 8-bitted Dribbble post
by UK artist and designer Harry
Harrison, San Francisco interactive
designers Addison Kowalski, Amadeus
Demarzi and Courtney Guertin took the
idea to the next level by 8 bit-ing
their Twitter avatars, which
inevitably went viral among the tech
set. Eightbit.me is the result.