Colour (Color) Contrast Check
I've improved it by adding some nice HTML sliders and revising the interface. I had also noticed a small bug in detecting my thresholds which has now (hopefully) been fixed.
For those not familiar, what the Colour Contrast Checker does is check two colours for, um, contrast. There's actually a formula that compares the difference in the colour and brightness.
It's important to have sufficient contrast to the colours on your page. Not only for people with colour deficiencies but also for people who are viewing it on screens that may not have great contrast, such as older CRTs or LCDs.
Lastly, this tool shouldn't be taken as gospel ("hey, look at me, I'm one of the disciples!") but rather should help guide you towards better colour choices. In the end, nothing beats actual user testing.