Update: These scripts no longer work. Both websites have since refreshed their user interface.
The del.icio.us Prettifier (right-click to install if you have Greasemonkey) changes the font, font size, and numerous link colors. The bookmarks themselves are highlighted much better than the default style.
The Last.fm Prettifier (right-click to install Greasemonkey) makes chart links utilize the entire width of the chart, and highlight in Audioscrobbler red on hover.