You can try UML tools. I would assume some have 30 trials.

>Basically, I need to create a class diagram. I was thinking is it possible without actually creating the classes and without having installed Visual Studio or Visio.
>
>There should be some other free graphical tool, but I'm not sure what to take. It should be something simple and not big.
>
>I need to get it done by tomorrow and I don't have tools installed at home.