Speed is a very important stat for any Pokémon.
Speed is how fast any Pokémon is. However the speed of a moving Pokémon's sprite slows down while its getting near fainting. Its a very important stat for any Pokémon.
Speed and Evasiveness are different.
Speed only determines which Pokémon will go first in a turn. Evasiveness is different. Evasiveness of your Pokémon depends how easy it is for the Pokémon to dodge moves.
Acupressure (By chance)
Ancient Power (By chance)
Ominous Wind (By chance)
Silver Wind (By chance)
Curse (Different for GHOST types)
Decrease opponent's speed:
Bubble (By chance)
Bubblebeam (By chance)
Constrict (By chance)
Secret Power (Real effect depends.)
Items that modify speed.
Choice Scarf (Multiplies speed, but lets the user only use 1 move until called back.)
Quick Powder (Only works with Ditto.)
Starf Berry (By chance. Can raise any stat at a time, its random though.)
Abilities that modify speed.
Increase Speed When Activated:
Lower Speed When Activated:
That's a whole ton about speed!
There are also a few juices (available in only X/Y) at the Juice Shoppe.
Pink Juice (Raises speed EV. If speed EV is maxed, increases Happiness. Obtain by mixing 2 Pink berries.)
Perilous Soup (Removes all EVs. Obtain by mixing Kee Berry and Maranga Berry.)
Source 1 (Speed raising and lowering moves, abilities, items)
Source 2 (Speed EV modifying Juices and all other things about the Juice Shoppe)
EDIT: Quick Claw, Trick Room, Lagging Tail also modify Speed.