Ben Kopf
Ben Kopf reviewedWhimsical WireframesLightning fast wireframing with real-time collaboration.
 RecommendedPros: 

super simple UX, fast, well thought out interaction model

Cons: 

none

This is the easiest diagramming app I have ever used. And the new wireframe feature is also extremely intuitive and great for quick ideas. While not as feature-rich as some others, it seems their intent was speed and simplicity over complexity, and they succeeded.

Ben Kopf has used this product for one year.