Gemify is a simple tool which helps you create and manage your gemspecs, but takes a much less obtrusive approach than Hoe, Echoe and Jeweler.
Temple is an template compilation framework for creating rock solid template engines.
SexpBuilder allows you to match and rewrite S-expressions in Ruby with some help of the excellent SexpPath.
Lately, S-expressions have been sneaking into Ruby. This dialogue attempts to explain the concepts behind it from a Rubyist's point of view.