The aDesigner is a disability simulator that helps Web designers ensure that their pages are accessible and usable by the visually impaired.
excellent tool, combining accessibility testing, simulation of vision impairments (including colour blindness), text-only/linearisation, readability...the lot.

you can get the download from (but you'll need to get a login if you don't already have one, but it's fairly painless)

As of now, there are no plans to commercially release the product, Chou said. The program was placed on the alphaWorks site to give developers a chance to use the application and suggest improvements. If there is enough interest down the road, the program could get wrapped up in the IBM product line.