FanFiction.net has changed its CSS so that visited links look the same as links you haven't visited yet. This is a very simple style that changes visited links to purple.
Also, this style makes it so that the advanced search options in the search page are always shown.

Edit 7/8/2012 - It looks like they've fixed the site now, so visited links change to purple. However I'm leaving this up as the links only change to purple no mater what default colour you set in your browser for visited links, and this script allows you to change the link colour to whatever you prefer as long as you know the right hex code (the #551A8B bit).

Edit 29/11/2012 - Removed my old update notes due to character limit. Updated it so that various Communities links should now change colour.