This one is really cool, I couldn't resist sharing it to you guys.
And I copied it from http://www.linuxforu.com/tips-tricks/install-linux-straight-from-an-iso/ :)

Now, let’s assume you have downloaded a new version of a distro and are in the mood to try it out right away. It’s past midnight and you realise that you’ve run out of blank CDs/DVDs. So you will have to wait till the morning when the shops open, to be able to burn the distro image in order to install it. I’m sure a lot of us often face this problem. In this article I’ll share a simple trick by which you can install the new distro without burning it to a CD/DVD. The only requirement is that you should have a pre-installed GNU/Linux system—which you already have, I assume.