Right now we have Mega Venusaur, Mega Charizard X, Mega Charizard Y, Mega Blastoise, Mega Beedrill (Complete special research: A mega discovery), Mega Pidgeot, Mega Gengar (Haloween), Mega Gyarados (Chinese new year), Mega Houndoom, Mega Manectric (Charge Up), Mega Lopunny (Easter), Mega Abomasnow.
I'll edit my answer when more become available