I think the key to prototyping is finding the right level of fidelity. That's going to be different for everyone, but the tendency is often to go too far. Seems like Quartz and Framer (although different) require a learning/configuration investment that only a small subset of folks would need to do in reality. Eg. honing the Facbook mobile experience, or innovating altogether new interactions.

At InVision we're working hard to strike the right balance and create the set of things (animations, transitions, etc) to carry and communicate the design concept well without slowing the process down.

I've used gimmebar.com, dribbble.com buckets, and a bit of pinterest to get into a diff stream.

InVision just launched some new stuff for mobile (transitions, gestures, etc) so combined with the desktop stuff, pretty useful. Use whatever tool to design, preview on any device.

I know some folks who use http://www.sketchup.com/ because it's easy to get started but also has some power to it.

InVision just launched fixed elements, transitions and gestures... might want to check it out again if you haven't looked at it in a while.

InVision has a free 1 project plan too.