UX design and wireframing to prototype interaction and colour rapidly.
UX Pin is the tool I use if I need to test some colour or layout as fast as possible, this is quicker than photoshop and I get the advantage that I can add interactivity to any element and create mock menu systems for example.
You can test an interface before you release.
Having the ability to send a preview to a client is a big benefit where comments can be made or a live presentation remotely administered.
How to Annotate your Designs Using UXPin