Lifegiver

Livegiver or Life Giver is a Fallout, Fallout 2, Fallout 3, Fallout Tactics and Lionheart Perk.

Fallout
With each level of this Perk, you gain an additional 4 Hit Points. This may not seem like a lot, but every little bit helps.

Although it's not clear from the description, Lifegiver gives you +4 HP when you take it; you could say it works retroactively for the level you just earned.

Fallout 2 and Fallout Tactics
Everytime your character gains a level, gain an additional four Hit Points for each rank of this Perk. With two ranks of Lifegiver, that’s +8 HP each level!

Although it's not clear from the description, Lifegiver gives you +4 HP when you take it; you could say it works retroactively for the level you just earned.

Fallout 3
With each level of this Perk, you gain an additional 30 Hit Points.

This perk also gives you unique dialogue choices when talking with some specific characters, like in the Oasis, Tree Father Birch.

Lionheart
With each level of this Perk, you gain an additional 6 Hit Points.