Pack Rat/Fallout: New Vegas

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

Fallout 2 and Fallout Tactics
The poor-man's version of Strong Back, as useless as the perk with the broad selection of party members and the Highwayman's huge trunk.

Fallout: New Vegas
The weight of all items weighing 2 or less is halved with this perk (2 becomes 1, 1 becomes 0.5, and so on and so forth). The perk is moderately useful in Normal mode, and is particularly useful in Hardcore Mode, where the multiplier applies to ammunition, allowing you to carry twice as much ammunition at the same weight.

Additionally, in Honest Hearts, the perk allows for bringing +25 pounds of gear into Zion (with the appropriate dialogue option with Jed Masterson).