desaturate.js is a javascript bookmarklet that desaturates all the colours on the current page, allowing you to see how clear things are to somebody who is colourblind.
This is an old project of mine and is currently very limited in what it supports, but eventually I hope it will be a useful tool for web designers who care about accessibility. For now, I recommend using Firefox, which has built-in tools for simulating colourblindness.