I've been a fan of ContrastRebellion, though I've lately simply taken to modifying websites' CSS that I visit often enough that it annoys me. E.g., I've swapped the border and content background colors on Hacker News.

What the heck, here it is: http://stylebot.me/styles/2945

I use Stylebot, but Stylish or other CSS management plugins should work just fine, copy/paste the CSS source.