dotjs is an extension that executes JavaScript files in ~/.js based on their filename and the domain of the site you are visiting.
If you navigate to http://www.google.com, dotjs will execute ~/.js/google.com.js. This makes it super easy to spruce up your favorite pages using JavaScript.
Bonus: files in ~/.js have jQuery 1.5 loaded, regardless of whether the site you're hacking uses jQuery.
GreaseMonkey user scripts are great, but you need to publish them somewhere and re-publish after making modifications. With dotjs, just add or edit files in ~/.js.
These are my own scripts that I use for various sites. You can use them if you want. They're MIT licensed.
cd ~
git clone git://github.com/skyzyx/.js.git
dotjs will automatically pick them up and start applying them. :)