The girdle or belt is named Megingjörð, and it is said to double Thor's strength.

The powers of the gloves Járngreipr aren't as clear, but they are listed next to Megingjörð and Mjölnir was his most important possessions, so they must do something cool. I've heard them being sent the hammer returns to his hand, or that it also doubles his strength - so combined his strength is increased fourfold.