ColorJack is a recently popularized Color Theory Application
They have a fantastic online system and an offline OSX Dashboard Widget. Basically, they have a whole load of settings which you chose from. You then drag one of the points around on the colour "sphere" (circle) and you are provided with a list of complimentary colours.
To try this out, get your browser over to the ColorJack Sphere where you can try it online.
So what is it really?
Well if you've ever tried designing a website, you might find it difficult to pick a set of colours which dont make you feel like vomiting or averting your eyes from the screen. I know I have (in the past) and its not nice. Some people find it very easy - but these people are natural artists whereas I know that I am not. I am a techie geek through and through and unless something is easily calculated or there is a logical path through to a conclusion, I find it quite hard to understand.
This tool is a godsend for anyone who needs to pick some colours for a website, so why not save yourself some time and get over to Colorjack and try out the free online tools (the Sphere and Studio) or, if you have a Mac (as all people should), grab the Dashboard Widget too!