Pack Rat

Pack Rat is a Fallout 2, Fallout: New Vegas and Fallout Tactics Perk.

You are efficient at arranging your inventory in general. This makes it much easier to carry that little extra you've always needed. This is useful for characters with low strength, i.e. sharpshooters and mediators.

Its usefulness is reduced in Fallout 2 with the addition of larger teams of traveling companions and the Highwayman's trunk.

Fallout New Vegas hardcore mode applies weight to ammo, which could cause this perk to be useful, allowing you to carry twice as much ammo at the same weight. Also of note is that this perk affects everything in the game world. Not just what is in your inventory.