Really love Balsamiq Mockups 3 - it's helped me build some awesome products. The shortcuts are super convenient and thorough, which makes prototyping like playing the piano. Amazing!
Ah! one shortcut I wish for - flip focus between objects on the workspace and the context menu (right) where I change sizing parameters.
simple to use; great shortcuts; comprehensive library of components
no overlays or states - each wire is like a static slide - makes you do more work