LiveConsole: A Window into Ruby
LiveConsole provides you with an IRB session running inside a thread in a running Ruby process. You can interact with it via TCP or a Unix Domain Socket.
Obviously, this can be valuable as a tool for doing arbitrary things, and it can also be a huge security hole if you aren't careful with how you use it.
I've found it useful for development, although your mileage may vary. Like most of my other code, there is a repo on github for LiveConsole if you want to check it out.
And if you are interested in LiveConsole, you may find Hijack more useful on systems where GDB is supported. It doesn't require you to include any extra code in your program or to manage file or network permissions.