HTMLInstant

I like using HTMLInstant for quick coding nothing serious, really.
You do not need to sign up but it's clear that the site probably hasn't had any updates in a good while.
On the left side you have a place where you can type your code and on the right side you have a place where you will see and be able to interact with what you've created as soon as you type on the left side.
It's very helpful when you need to see what something will look like but please do not rely on this to create a full website. It's a great tool for anyone creating layouts for sites like Vampirefreaks.com or something.