Mockingbird is a fantastic in-browser tool for creating website wireframes
Mockingbird is like a digital version of the back of a napkin, but on steroids. It runs in-browser, and is powered by the magic of HTML5 (so it might work on an iPad). Mockingbird provides you with a rich toolbox of UI widgets like buttons, rectangles (both rounded and "regular") and image placeholders, as well as more Web-specific elements like a "search box" or vertical/horizontal navigation bars.
All you have to do is drag-and-drop the elements around the screen and resize them until it looks just right. I've tried it with one website mockup, and the result was pleasingly clear – it communicated exactly what I had in mind.
Mockingbird is a paid service (starting at $9/month) but you can use the tool itself even without an account – you just won't be able to save your work. That's not as bad as it sounds, because more often than not you may just want to relay a quick idea.