Viewflux - Feedback, prototyping and collaboration for designers
ProjectHuddle - Get design feedback and client approval using WordPress.
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.
Invision - Prototyping and collaboration for design teams
Notism - What's next after wireframes? Visual Design and Notism.