Can anyone recomend me a simple web UI prototyping tool, so I could quicky prototype the look of a my web site. I have tried to use MS Visio for this, but found it very "user un-friendly". What I really need is to be able quicky sketch the layout of the page, put some links, images and buttons on in, play a little bit with a colors (CSS), and it would be great it this tool could support navigation between the pages - but it is not essential. I would rather consider a low-cost or an open-source solution, since I am not a web designer and not going to use that tool on a daily basis.