They are mainly used within the ruote open source Ruby workflow and BPM project, but some of them have a life on their own, for example, the scheduler is used in Taskr a RESTful cron/at job scheduling server.
The project page on RubyForge : http://rubyforge.org/projects/rufus (download)
Since March 2008, the source code has been moved to github (source)
The mailing list for all these gems is at http://groups.google.com/group/rufus-ruby
The author’s blog : http://jmettraux.wordpress.com