For the developers out there that sometimes like/have to pretend they are designers and need to create prototypes for graphical user interfaces the Pencil Project is an application to take in mind.
For starters, it’s free, unlike Microsoft SketchFlow or Balsamiq Mockups, which judging by the demo at their site seems pretty interesting but since I do some pretty basic stuff I don’t think it would pay off. Have to check their trial, though.
The Pencil Project runs as a standalone application or as a Firefox add-on. You can download it at the main site or directly from the evoluspencil code repository where you can also find more stencils packages.