Stylizr

Insert custom CSS into pages, immediately see the visual results, and have that CSS persist for future visits.
Usage:
You may dislike a given site's design and want to add your own touch--that is, your own style--to make it aesthetically pleasing. After installing the extension, visit the site (e.g. techcrunch.com) and hit control + m. You should see a textarea to the right of the browser window. Insert your custom CSS into it and view the results.

This CSS will be reinserted upon revisiting the site, thereby saving your styles. Pressing control + m once more will hide the textarea, but your contents will still be retained.