RWD Wireframes (github) is a small scale, browser based wireframing tool for responsive layouts. The tool allows you to define a few containers and then determine their size, placement, layer order, and visibility for various screen widths (which of course may also be manipulated). Apparently once an account is created you may also share your work in the form of links. Oh and it’s open source as well so you may fork it and tweak it github style. Thanks Hao for sharing this innovative project and pushing some boundaries on version 0.0.1!