Note that the debugger and the profiler are part of the standard distribution.
The Ruby debuggerThe library debug.rb is invoked by a -rdebug on the command line; this gives access to a command-line debugger similar to gdb and its ilk.
YARD (Yet Another Ruby Debugger)This is an enhanced debugger that can be used to attach to programs running remotely.
The Ruby profilerUsing -rprofile on the command line will produce profiling output detailing numbers of method calls, time in seconds, and so on.
RubyUnitThis is a unit-testing framework based on junit.
LapidaryAnother unit-testing framework, this based on sunit instead.
RubiconThis test suite is primarily useful for verifying that the Ruby installation behaves as expected. If you use development versions of Ruby (rather than stable versions), you might want this package.
swigrubyThis is a Ruby-aware version of SWIG (Simplified Wrapper and Interface Generator), which glues C/C++ to Ruby.