I really like JSF 2, especially when integrated in the whole J2EE stack, but component libraries like RichFaces or PrimeFaces can be somewhat heavy on the front end.
So, I decided to try out Twitter's Bootstrap UI library with just plain JSF 2, without any component libraries. For testing purposes, I modified JBoss 7 skeleton maven archetype project to use Twitter Bootstrap, and the two seem to be getting along nicely together.
This could be a nice alternative for building lightweight JSF 2 web applications, especially if you would build a good base of common composite components.
One of the advantages of Twitter Bootstrap is that it comes pre-optimized for mobile devices:
You can check out the source code for this on my GitHub: https://github.com/rkovacevic/JSFBootstrap
I tested it on JBoss application server 7.1.