The mini-debugger is a very simplistic memory debugger built into the mac os. You get it if you deliberately invoke MacsBug on a computer that does not have MacsBug installed (depending on which of the following is possible with your model of mac, do one of the following: press the programmers key on the case, or hold the command key and press the keyboard power key, or hold the command key and press the keyboard cd eject key). You sometimes hear the mini-debugger referred to as "that weird box with the >", and if you ever hear it referred to that way you will almost invariably be next asked the question "how the hell do i get out of that thing?". Almost nobody knows what it is, and very few of those who have come into contact with it are aware it has a name. Almost everyone who has ever come into contact with it invoked it accidentally. While originally intended as a simple debugging tool, its main purpose tends to be to confuse the shit out of people.

The first thing you will want to do upon entering the mini-debugger is leave. This can be accomplished by typing g and hitting return.

There is exactly one useful thing you can do with the mini-debugger, and that is force quit applications. It is possible, instead of typing "g", to type "g f" or "g finder" to force quit and return to the finder. This comes in handy sometimes, as a mini-debugger force quit is marginally more likely to work than an command-option-esc force quit.

Somewhere, somehow, documentation for this beast must exist, but God knows how to go about finding it.

The only command other than g i am aware of is sm, which i assume stands for "show memory", and can be followed by a hex address. sm basically dumps the entire contents of memory as hexadecimal values, line by line.
This command was discovered by this guy i know named Sean, who is not a mac user, but is a complete moron. Upon being confronted with the mini-debugger on a mac at school, he decided to type in SMAK ME, thus invoking the sm command. Go figure.