This is a great web design tool. I don't know how many times I have wanted to replicate a color from a printed piece for the web, and had to go into photoshop or another graphics program to convert the color into the proper Hex code. Which isn't a huge deal, but this site makes it a whole lot easier. Colours should be tested across different platforms, operating systems, browsers and versions to ensure colour is reliable displayed in typical viewing environments, since not all systems are the same.
Check it out here.