Balsamiq Mockups alternatives

Balsamiq Mockups
Mockups really shines during the early stages of designing a new interface. Mockups offers the same speed and rough feel as sketching with pencil, with the advantage of the digital medium: drag & drop to resize and rearrange elements, make changes without starting over, and your work is clear enough that you'll make sense of them later.
Balsamiq Mockups alternatives are:
Omnigraffle, Axure, MockFlow, Wireframe Sketcher, Microsoft Visio