Notism - What's next after wireframes? Visual Design and Notism.
Invision - Prototyping and collaboration for design teams
Red Pen - Red Pen lets you upload your design, share a short URL, and get live, annotated feedback super-fast.
Pastel - Sticky note-based feedback collection tool for live websites
Notable App - The Markdown-based note-taking app that doesn't suck.
Viewflux - Feedback, prototyping and collaboration for designers