Screely - Screely instantly generates dribbble-ready images. Simply upload your screenshot and we'll do the rest.
POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
Pika App - A free tool to quickly generate beautiful browser screenshots. You can capture screenshot from website, add browser frames, 3d tilt-effect, background colors to screenshots and more
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Screenzy - A simple screenshot beautifier
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.