Glui - glui - GLUI is a GLUT-based C++ user interface library which provides controls such as buttons, checkboxes, radio buttons, and spinners to OpenGL applications. It is window-system independent, using GLUT or FreeGLUT.
Savahapp - The all-in-one design sharing, collaboration, workflow and app prototyping platform
Red Pen - Red Pen lets you upload your design, share a short URL, and get live, annotated feedback super-fast.
GoodUI Fastforward - Lift your conversion rate with repeatedly AB tested patterns
ProjectHuddle - Get design feedback and client approval using WordPress.
FLTDSGN - Daily showcase of the best flat UI design websites and apps