quickMockup - Online tool for creating mockups in the browser.
ProjectHuddle - Get design feedback and client approval using WordPress.
POP (Prototyping on Paper) - POP helps you make interactive prototype with ease. If you can draw, you can design apps.
Savahapp - The all-in-one design sharing, collaboration, workflow and app prototyping platform
Invision - Prototyping and collaboration for design teams
Specfox - A free platform for creating beautiful website and mobile app specification documents for your team. No more word templates!