Cycle, toggle, change stances using keybindings. Create different classes of spells to be with a single key press.

WARRIOR
http://www.darthnader.com

Warrior is what I like to refer to as a smart keybinding mod. Meaning you can bind a single key to perform different actions based on a set of conditions or spell availablity. There are many other mods out there at provide smart keybinding functionality. The defining difference Warrior has is that it leaves the condition building for which spells should be cast completely to the user. So a user who always Charges, Rends, and then Hamstrings can roll those actions into a single keybind (it is important to note that this is NOT a single keypress it is a single keybind).

In my opinion this is functionality Blizzard should have built into the game. By rolling up 10 keybindings / action slots into 1 keybinding / action slot you remove an unneeded level of complexity and allow yourself to better enjoy the game.