Marvel provides a quick way to send a live prototype to clients or colleagues, even if you've only made a sketch of your interface ideas.
All you need to do is upload each screen of your site or app to Dropbox and use Marvel's simple interface to link them together. To make a button clickable, you draw a box over it and specify that it's a 'hotspot', then indicate the screen it should link to. You can even add animations such as a page-turn effect to your transitions.
Marvel doesn't ever edit your Dropbox files, and they're always synced - so if you make a change to your source files it will carry through into the prototype. It works with JPGs, PNGs, GIFs and PSDs.
Prototypes are shared via a unique URL, and mobile apps display in an iPhone simulator. Marvel app is free to use.