CSS Stats Bookmarklet
CSS Stats is a useful tool for getting a bird's eye view of the state of your website's CSS. By itself, it isn't anything particularly groundbreaking, but using the information it provides, you can get a feel for the quality of the CSS you have written.
I made a nifty little bookmarklet that runs the current page through CSS Stats using the default user agent. To use it, create a bookmark that contains the code snippet below.