When Sceptile mega-evolves, Sceptile becomes a grass/dragon type Pokemon and gains the ability "Lightning Rod". The name for the mega-stone has not yet been confirmed but I believe it will be called "Sceptilite".
Mega Blaziken is the same as in Pokemon X and Y. When Blaziken mega- evolves, Blaziken becomes a fire/fighting type Pokemon and gains the "Speed Boost" ability. Blaziken's mega-stone is called "Blazikenite"
When Swampert mega-evolves, its type (water/ground) stays the same and it gains the "Swift Swim" ability. There is not a official name for it's mega-stone however , I believe it will be called Swamperite.
It has not yet been confirmed if "Primal" Groudon(and Kyorge) are megas yet but, they do have a new form. Not much is know about them yet except for their names and texture(see above picture).
When Sableye evolves, Sableye's type changes to dark/ghost and gains the ability "Magic Bounce". When Sableye is mega-evolved it gains a big boost in Sp.Attack and defense. However, it does slightly decreases it's speed.
When Diancie is transferred to Alpha/Omega you will somehow(not yet known) get "Diancite". Not much is known about Mega-Diancie at the moment except how it looks(see above image).
This was all for Mega-Evolution reveals so far. However, mega Latios and Latias may be in Alpha/Sapphire since they are in Pokemon X and Y. Next up, many screenshots were shown at E3 including gyms, team Magma/Aqua commanders, e.t.c. Here are the screenshots from E3:
*Pokemon Omega Ruby and Alpha Sapphire will be released on 3DS November, 2014*