NormalMapper, a standalone normalmap-generation tool for Windows, hasn’t been around for very long.  However, they just recently decided to go free & donation supported.

Introducing NormalMapper, the first standalone tool to create and modify normal maps as a native object instead of another color layer. NormalMapper can generate directly from a source image or can be used to create from scratch or modify existing normal maps via tools that manipulate the normals as 3d vectors, not color channels.

You can download NormalMapper at their website.

via The Official NormalMapper Website.