I'm a little biased (I'm the author of webgen :) but I think webgen would suit your needs. You can install it via `gem install webgen`, it runs on Ruby 1.8, 1.9 and JRuby, supports Markdown/Textile/Haml/ERB/... and is easy to use:

* Create your website:

    webgen create my_site

* Render your website:

    cd my_site; webgen

The built website uses only relative URLs, so you can preview it by just opening `my_site/out/index.html`. Adding a page is as simple as adding a file with the extension page to the my_site/src directory. The styling of the site is done via templates, the main template being my_site/src/default.template. You can select from several different styles when creating a website (see `webgen help create`).

Menus, breadcrumb trails, links to translations of the current page are all automatically generated.

You can, if you need/want to, use webgen from a Ruby script:

    require 'webgen/website'
    ws = Webgen::Website.new(ws_directory)  # ws_directory would be the my_site directory in the example above
    ws.render

For more information have a look at the webgen website.