ModMod allows you to bind new key combinations by creating custom "modifier" keys.

Note: This addon is in maintenance mode. Use /modmod to summon the binding window.

The traditional modifier keys, such as Shift and Alt, allow you to bind Shift+W to open your bags and Alt+E to auto run.

If you turn Q into a ModMod key, you can bind Q+W to open your bags and Q+E to auto run.

How to Use

Key combinations are created in the ModMod window which you can summon with the /modmod command.

In the left column are the new modifiers to bind, the ModMod keys. You can create up to 10 of these.

In the right column are the actions you can bind while the ModMod key is down. These are independent of all other binds and you can bind them all to a ModMod key if you want!

You can also bind mouse buttons to be either a ModMod key or an action. For instance:

If Button5 is the ModMod key and Q is bound to Jump, then while Button5 is down, Q will jump.
If Q is the ModMod key and Button5 is bound to Jump, then while Q is down, Button5 will jump.

Limitations

There are a few limitations to this style of binding:

ModMod keys cannot be used as a [mod]ifier condition in macros.
Nor does this allow a ModMod modifier key to be used for non-ModMod purposes. You can't have Spacebar be a ModMod key and continue to jump with it. (But you can bind Button5+Spacebar to something else while Space continues to jump.)
This works with all addons that expose their binds to the key binding interface. Addons that don't list their binds are hidden from ModMod and it's up to the addon to list their binds for ModMod to support them.
I recommend not using this to bind actions that start and stop, like movement. Releasing the modifier before the bound key will mean you won't stop because it never got the "upstroke" event.

Otherwise, use your imagination and have fun experimenting!