Quoting its presentation, "Jekyll is a simple, blog aware, static site generator. It takes a template directory (representing the raw form of a website), runs it through Textile or Markdown and Liquid converters, and spits out a complete, static website suitable for serving with Apache or your favorite web server."
Jekyll is It’s github's Wiki engine. This is a clever integration: the VCS tracks Wiki page changes, which means the project hosted by github has a wiki and versioned documentation along with sourcecode.
Jekyll is a batch-only generator, so no interactive preview. Markdown and Textile are also more lenient about grammar validation. But Jekyll is probably much lighter than Novelang, which makes it a better choice for github.