This style is similar to, and inspired by, marcanth@userstyles's theme http://freestyler.ws/style/132/check-marking-visited-links-with-custom-color. The biggest difference is the change to visited links is more noticeable and doesn't distort the layout of pages.

I chose a nice blue because it worked best for my needs but it isn't hard to change if you don't like the blue. Just replace each instance of "#00cccc" with the color of Your choice

June 14, 2012:
Does not work for firefox version 4+ and as far as I know there is no way to make it work. I do not know if it works for other browsers.

September 22, 2012:
Updated to work with ff15. Color can be changed but must be a color name, hex defaults to white, did not test rgb.