Visited
Why on earth didn’t I think of this myself? It’s so simple to do, yet incredibly elegant. Here’s what:

That’s the list of recent del.icio.us links over at Lisa McMillan’s site. When I first visited I wondered why those two links had marked checkboxes, and when I realized they were links I had visited earlier I instantly started to mentally slap my forehead for not thinking about that myself.
I’m totally stealing that idea for whenever I design my own theme for WordPress. It’s very simple to do: make a list with unvisited links having one background, and visited links another background. It looks much more special and attractive than changing the link text alone.