AppTaster - Share mockups and wireframes with clients on their iPhone or iPad.
quickMockup - Online tool for creating mockups in the browser.
POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
Invision - Prototyping and collaboration for design teams
Composite - Composite is a brand new way of creating interactive prototypes. It automatically connects to your Photoshop® document and converts your mockups into interactive prototypes in seconds. No need to export images or maintain tons of hotspots.