Did you know about it?!
The Developer console has been around for a long time[1], but was only made available to players with the release of Z-buffering on 15 September 2010. It may, according to Jagex, "prove useful in tracking down the cause of more technical issues with the client on your computer." You can view a list of commands by typing either help or commands in the window. It also displays Runescape's Build Number in the lower right corner.
To get to this console press the [`] (quote/grave accent) . It is found to the left of the '1' key on most English keyboards. The tilde [~] key on your keyboard is another way to open it. If you find yourself unable to close it, try holding [shift] while pressing any of these buttons.
This is the wiki link with command list.