This sweet little Chrome extension allows you to make web apps such as Google Mail and Google Reader more minimal than they already are.
When you're using it in Gmail, you can customize what you see on screen heavily. You can hide things like the dark grey bar which stays at the top, hide the search bar, hide the logo and lots more. You can even change colours of things which you normally wouldn't be able to do.
It's for the extreme minimalist or even for people who might find a particular thing in a web app annoying.