How to force coloured display of visited links in Safari 5+?
You could create a text file, name it visited.css, and put this text in there:

a:visited { color: #6611cc !important; }


Then go to Preferences > Advanced, click Style sheet and select the .css file you just created. All links you've visited should be in a nice purple now.
(Note that this colors every link you've ever clicked purple, and will override the actual web design.)