Generation Iv
Unburden
Raises Speed if a held item is used.
Detailed Effect
When this Pokémon uses or loses its held item, its Speed is doubled. If it gains another item or leaves battle, this bonus is lost. This includes when the Pokémon drops its item because of Knock Off. This bonus does not count as a stat modifier. There is no notification when this ability takes effect.
Pokémon With Unburden
17 canonical Pokémon forms can have this Ability.

Drifloon
Drifblim
Purrloin
Liepard
Hawlucha 
Nickit
Thievul
Sneasler
Shroodle
Grafaiai
Hitmonlee
Treecko
Grovyle
Sceptile
Accelgor
Swirlix
Slurpuff