RJudy was a Ruby extension module that provided an interface to the Judy arrays library. It is no longer maintained, but the sources are provided here for historical purposes.
Judy was originally implemented at Hewlett-Packard, and was subsequently open-sourced in June 2002 (with an LGPL license). Doug Baskins, the author of the Judy algorithm, has written a nice summary of what makes Judy arrays so fast, and why you might want to use them.