I have a submission.
canvas.pushDialog(GUI_Name);
Opens one of the game's GUI's in-game, where "GUI_Name" is the name for it (the names can be found by opening the GUI editor).
For example, pushDialog(AvatarGUI) would open up the avatar customization screen, pushDialog(JoinServerGUI) would open up the "Join a Game" window, pushDialog(RTB_ModManager); would open up the RTB mod manager, pushDialog(KeyGUI) would open up the screen that lets you enter your authentication key.