“Your skills have improved to the point where you can pick an additional Tag skill, increasing that skill by +20% immediately and 2% per skill point spent.”
Fallout: Permits a fourth tag skill, adding +20% to it and doubling the rate at which it increases.
Fallout 2: Permits a fourth tag skill, doubling the rate at which it increases, but does not add anything.
In Fallout and Fallout 2, this perk also doubles all skill points previously spent on the selected skill (but not any skill points added by perks).
Fallout 3, Fallout: New Vegas
“The Tag! perk allows you to select a fourth skill to be a Tag skill, which instantly raises it by fifteen points.”— In-game description
Because of the reduced properties of tag skills, this is essentially nothing more than a fifteen point bonus to any given skill.
While tagged skills do not have the same benefit as in previous games, this perk is the highest skill bonus perk in the game, along with the perk Size Matters, as all others are +10 (not including Impartial Mediation, as it requires neutral karma and therefore is not always active). It is useful for quickly bringing a new skill up to the level of the starting skills, e.g. 60 Lockpick becomes 75, enough for a hard lock, and then the skill points can be added to make it 90.
This perk becomes more useful in Fallout: New Vegas due to the lower number of skill points received when leveling, the lack of skill Bobbleheads, and fewer skill books available. Tag! is equivalent to an extra level's worth of skill points with maximum Intelligence, not counting the Educated perk.