Bagnon is a wonderful addon that groups all your UI's bags into one visually pleasing ensemble, making sure that the player doesn't lose some much needed vision due to something as simple and banal as opening heir inventory. There are many alternatives with similar effects, but out of all the bag addons, Bagnon would have to be my personal preference.
Features:
Unneeded "bags" - or rather their slots - such as the one for keys, can be hidden with the press of a mouse button.