Red is red, right? Wrong. There are some people out there (this humble blogger amongst them) who are color blind in one degree or another (I have a slight case of red/green colorblindness). This means you can't assume people see colors in the same way you do, which is very important when you're a designer.
But what can a designer who sees in full color do in order to make sure their websites are legible to all those colorblind surfers out there? That's where Sim Daltonism comes in. This little donationware app simulates what the area around your mouse looks like to a person with a variety of different types of colorblindness.