Action group overview.

[Aeroboi]

In stock KSP you normally set action groups for your vessel and then you have to write down the set action groups for you to remember what they are. Of course, this can be done... Otherwise you could use "action groups extended" mod. You wont always want this because you want to have a stock game making stock craft.
In stock KSP you won't have any in game notification what part action groups you set under 0 through 9.
Therefore, maybe the stock game could use a in build function attached to the toolbar or anywhere else for that matter so you have information how action groups are setup.

[zer0Kerbal]

Action Group Manager Renewed

[Pthigrivi]

True. And it might as well be editable so we can add and remove things from the action groups in flight.