WordPress: Open source blogging/CMS.
Blogging for a Good Book: Sample WordPress site (hosted).
LibraryThing: Catalog your whole library online.
Scriblio: Open source OPAC/CMS.
Lamson Library: Sample Scriblio site.
Drupal: An engine suitable to setup or build a content driven or community driven website. Modular design allows flexibility in design.
Franklin Park Public Library: Sample Drupal site.
Fish4Info: a next generation social alternative library portal. (See this for more info.)