I'm not a designer by training -- in fact, I can barely draw stick figures. But often on a project I need to start laying elements out on a page and seeing what works, and what doesn't belong. I used to do this on paper, which is quick and easy to iterate. Then, start mocking up the layout in HTML. All this is just to get the features and placement right, and leaving the visual design for a professional.
The problem with this approach is, there's a huge gap between paper-prototype and HTML mockup. Paper is quick and easy, but not very compelling to look at. HTML looks better, but it's not very malleable -- once you place something, it's often a pain to move it around and get it right again.
So I've started using a drawing tool to do these wireframes. I may sketch something out on paper first (because that is still the fastest), but before I share it with anyone, I'll open up my drawing program and draw boxes and text on the screen. I'm on a Mac, and so far the best tool I've found for this is OmniGraffle ...