lg is much more than just a data inspector, it's a full-blown REPL. The fact that you can jump in and interactively eval code directly in the running process is huge and opens things up to a whole new level of hackability.

And there is a DBUS interface for it, so you can even push code into the shell from external processes.

The js extensions are fantastic. It's a real shame there is little (no?) documentation to help people get started.