Gen I:
Gen II:
Gen III:
Gen IV:
Gen V:
None.
Gen VI:
Unconfirmed:
Rumoured:
Note that Gen VI Pokémon will not get Mega-Evolution. As for now, Galvantula may be the only Gen V Pokémon able to Mega-Evolve.
Hope this helps.
Venasaur Charizard (2 forms) Blastiose Mewtwo (2 forms) Gengar Banette Alakazam Kagashkan Pinsir Aerodactyl Gyarados Heracross Ampharos Scizor Tyranitar Houndoom Gardevoir Blazekin Manectric Aggron Abomasnow Mawile Medicham Lucario Absol Garchomp
Here is a list with some pictures of Mega Evolved forms
The actual list:
Source