Underfoot

Paragon Tier
Prerequisite: 11th level, Halfling, trained in Acrobatics
Benefit: You can move through the space of a creature two or more sizes larger than you (Large or larger) without provoking opportunity attacks from that creature. You don't provoke when you leave an adjacent square to enter the target creature's space or when you leave the target creature's space to enter an adjacent square.
    You still provoke attacks from other creatures. You can't end your move in another creature's space.

Published in Player's Handbook, page(s) 206.