Belt of Storm Giant Strength
The description of the Belt of Storm Giant Strength states (DMG, p. 155; emphasis mine):
Wondrous item, legendary (requires attunement)
While wearing this belt, your Strength score changes to 29. The item has no effect on you if your Strength without the belt is equal to or greater than the belt's score.
This belt is the only one I know of, and it only sorta qualifies: the belt sets your Strength modifier to +9, so if you consider a +5 Strength modifier as the baseline, then the belt is equivalent to a +4 to all Strength-based rolls, including attack rolls, saving throws, ability checks, and damage rolls.
Of course, it's possible for a character's Strength modifier to be higher or lower than +5, but the vast majority of characters that need Strength reach that +5 threshold as early as possible and then never go higher, so it's reasonable to consider the belt as a +4 above that norm.
Many characters substitute the Strength modifier in their attack and damage rolls (i.e. finesse). For these characters, we can still assume that their relevant ability modifier is +5, so the improvement to their attack and damage rolls is still 4, but for these characters the benefit to Strength-based saving throws and skill checks is greater than 4.
Moreover, the belt's benefits are cumulative with the typical +X granted by magic items like weapons.