Unburden

When a Pokémon with this ability consumes an item or has one removed from it (for example, by Trick, Knock Off, Bug Bite, Fling, or Natural Gift) its Speed is doubled until it switches out. This boost is not a change to the Pokémon's Speed stage, so if a Pokemon used Agility after Unburden activates, it would have four times its original Speed. The Unburden boost cannot be Baton Passed, and losing items after the first does not provide additional boosts. Regaining an item does not remove the boost.

Found normally

Pokémon	Type	Tier	Abilities	HP	Atk	Def	SpA	SpD	Spe	BST
Drifblim	Ghost / Flying	NU	Aftermath / Unburden / Flare Boost	150	80	44	90	54	80	498
Drifloon	Ghost / Flying	LC	Aftermath / Unburden / Flare Boost	90	50	34	60	44	70	348
Liepard	Dark	NU	Limber / Unburden / Prankster	64	88	50	88	50	106	446
Purrloin	Dark	LC	Limber / Unburden / Prankster	41	50	37	50	37	66	281

Found in Dream World

Pokémon	Type	Tier	Abilities	HP	Atk	Def	SpA	SpD	Spe	BST
Accelgor	Bug	RU	Hydration / Sticky Hold / Unburden	80	70	40	100	60	145	495
Hitmonlee	Fighting	RU	Limber / Reckless / Unburden	50	120	53	35	110	87	455
Sceptile	Grass	RU	Overgrow / Unburden	70	85	65	105	85	120	530
Treecko	Grass	LC	Overgrow / Unburden	40	45	35	65	55	70	310