This is my HTML
<div class="one">...</div>
<div class="two">...</div>
<div class="three">...</div>
<div class="four">...</div>
<div class="five">...</div>
How can I get this image by using only CSS? I guess with float, but how can I get the fifth div next to the first one?
Changing the HTML is NOT (!) an option.

My first comment would be that class names can’t start with a number, so I really hope that you can edit the HTML for that. To answer your question ignoring this fact, if each element has a class, this is pretty simple. Just do this:
Preview: http://jsfiddle.net/Wexcode/mvwSL/