If you're a Firefox user and savvy about CSS selectors, I recommend the Stylish add-on (http://userstyles.org/). It lets you create blanket tweaks for all sites and specific tweaks for specific sites. And you can turn your tweaks off and on at will. It works even with Javascript turned off. I set colors and fonts and font sizes, adjust column widths, eliminate entire columns, make images semi-transparent so they don't bug me so much, etc.
Also, if you're an opera user or have greasemonkey the styles can be loaded as userscripts, which will just use javascript to edit the css style changes you have selected.
