MetaRuby contains miscellaneous libraries (useful now) for a future Ruby-in-Ruby interpreter including Array/Hash/String as abstract ("Hollow") classes, an undo queue, a statistical time-profiler, an event loop, a modular marshaller ("ToSource"), a specification for a modular+reflexive+homoiconic remote call system ("LGRAM"), a declarative type system, a schema for expressing Ruby source code as proper (non-special) Ruby objects, etc.
REXML is an XML 1.0 compliant, reasonably fast, non-validating XML parser. It has an API that is designed to be intuitive, straightforward. and terse. REXML includes a tree model parser, a SAX2 streaming parser, and a pull parser. It also includes a full XPath implementation. All of REXML's parsers pass 100% of the Oasis XML non-validating tests. As of Ruby 1.8.0, REXML is distributed with Ruby.
Ruby-GConf allows Ruby developers to write applications that store their configuration data using GConf (1.0.x). GConf is a configuration database system written by Havoc Pennington et al, and part of the GNOME application development framework (although it can be used separately).