Custom command keybindings in Gnome

Gnome (or, more precisely, Metacity), will let you create 12 custom key bindings. That is, you can set 12 custom commands that are triggered by a given key sequence.

This is done in Configuration Editor (System Tools -> Configuration Editor; enable it in "Edit Menu" if it isn't there). The first custom command is set with /apps/metacity/global_keybindings/run_command_1. The associated keybinding is set with /apps/metacity/keybinding_commands/command_1.