I found this accessibility toolbar from Vision Australia a few years ago and use it all the time: http://www.visionaustralia.org.au/ais/toolbar/
The toolbar, which is updated all the time, allows you to do standard accessibility validations plus a host of other tests. I really like the simulations that show how web pages look to folks with various forms of visual impairments.
Give it a try. Let me know what you think.