I have being doing this for a while using Balsamiq Mockups 3. The fact that everything is rendered as a low fidelity mock really helps avoid discussions like "

I have being doing this for a while using Balsamiq Mockups 3. The fact that everything is rendered as a low fidelity mock really helps avoid discussions like "move this 2 pixels to the left" or "maybe this blue should be more towards green". The exceptions are set in wireframe and when we have a working version is a nice surprise.