Whats a good prototyping tool for designing Web Layouts?

Hi all,

I need a tool with which i can design Web Application page layouts easily. Can anyone give some suggestions?

I found this one:
http://www.microsoft.com/expression/products/sketchflow_overview.aspx
but i’m not sure if this is indeed the most powerful out there. The story is: our client does not want to work with documentation, our client wants to work with “layouts”. We now need a tool in which we can easily create these layouts and send them to the client so that they can later approve and/or change (using the same tool) them.

Thanks,
Gabriel