I've dabbled with wireframe tools before such as mockingbird but have recently plumped with the Invision App and really like it. It's more about sharing and commenting than actually creating which our designer can do easily do in Illustrator or Photoshop. Users don't have to register to comment and as it's a private link it doesn't really matter.
Just thought I'd share - we're not on commission...
Just thought I'd share - we're not on commission...