I played around with a rather nice wire-framing tool today. Wireframe.cc takes a minimalist approach to its interface and user interaction. Simple elements and a limited color palette keep your wire frames clean and allow faster iteration. You can layout browser, tablet, and mobile screens. Easily share your wire frame with a generated URL, or bookmark it to come back and edit later. And all that for free.
I like that it adds just enough polish over my hand scribbled mockups but is still fast and easy to work with.