It slices, it dices, it generates static sites, it natively supports preprocessors like LESS and Stylus:

Imagine you were choosing between PNG or JPEG for an image you wanted to serve. You would simply pick the right format for that use case because web servers support either, you need only drop the file in. That is exactly what Harp does with modern web languages. Want to use Stylus? Just name your file with a .styl extension. Prefer LESS’ syntax? Just drop in the LESS source files and go to work. Or use a combination of both, Harp doesn’t care. Harp knows to compile and serve main.styl when main.css is requested, OR main.less if that file exists instead. It’s that simple.

That’s a hell of a pitch, and it’s only about 1/3 of the whole thing. Read the whole post.