Segment.io's Metalsmith doesn't excite me because it's a static site generator. It excites me because its everything-is-a-plugin philosophy turns it in to a potential swiss army knife for any project that manipulates a directory of files.
It could be a project scaffolder. It could be an e-book generator. It could be a build tool. It could be a documentation tool. It could be something I've never even heard of before.
Check out their examples to whet your imagination.