Check out these awesome user interface sketching tools from UI Stencils. Here’s a browser sketch pad, and a web interface stencil that’s scaled to suit the pad perfectly.
The web elements stencil features a pixel ruler. While obviously pixels aren’t real measurements off-screen, it does provide you with a scale reference for working with the sketch pad.
Here’s the iPhone interface stencil, built to iPhone 4 dimensions and scale so you can map out your apps at the size they’re actually going to be built for:
And finally, an iPad dry erase board for quick and dirty interface brainstorming sessions.
If you don’t want to shell out real money for wireframing tools, there are plenty of free alternatives that you can print out–though you will be missing out on the metallic goodness of a real stencil!