Webby (http://webby.rubyforge.org/)

Branch with ruby 1.9 fixes is at https://github.com/namelessjon/webby

It is not under active development, but is for me feature-complete, stable and incredibly flexible. Advantages are:

- pure ruby (no liquid templates needed)

- each page has its own subdirectory where one can put images, scripts etc; your article becomes a 'package'.

- chained templating filters; you can do erb->markdown->your custom filter with minimal effort.

- mostly implemented by extending rake tasks, no extra api to learn and if you know rake you can continue adding features forever.