The 30 Second Guide to Ruby Command Line Debugging
Chad Woolley has put together a helpful blog post called "ruby-debug in 30 seconds (we don't need no stinkin' GUI!)" that covers how to use the ruby-debug command line debugger to debug Ruby applications. Also of interest will be this cheat sheet by the guys over at ErrFree.
If you're not into GUI editors or haven't found a GUI Ruby debugging solution, it's worth looking into ruby-debug. Chad says now that he actually prefer this "IRB-style" of debugging to the graphical alternatives.