What is it?
Vischeck is a way of showing you what things look like to someone who is color blind. You can try Vischeck online- either run Vischeck on your own image files or run Vischeck on a web page. You can also download programs to let you run it on your own computer.
What is it good for?
Roughly 1 in 20 people have some sort of color vision deficiency. The world looks different to these people: they often find it hard to tell red and green things apart. This often means that they sometimes can't see things that 'color normal' people can see (examples).
Many pictures, documents and web pages are hard for color blind people to read because the people who designed them didn't think about the problem. Vischeck lets them check their work for color blind visibility. It is also interesting to anyone who is just plain curious about what the world looks like if you're color blind.