Hmm...Hopefully I won't miss anyone. Note that I'll even include those from the spin-off Manga , Nightmare of Nunally or other form of spin-off characters who uses the Geass.
Firstly...the almighty Lelouch!
Lelouch vi Brittania :
As in Anime :
In the Anime , Lelouch obtained the Power of the Kings , the Geass that allows him to give an absolute command that no one can ever defy. However , the limitations of his Geass is that his Geass is affected by light , thus , eye contact , however , glasses won't affect it. Another limitation about his Geass is that he cannot withdraw his order and it can only be used once per person.
Contractor : C.C.
As in Nightmare of Nunnally :
In the Manga , Nightmare of Nunnally , Lelouch gains a very different type of Geass. Unlike the Geass that gives him the ability to control , he gains the Geass that gives him the ability to fight. Whenever he uses his Geass , he will be clad in an impenetrable armor that can even match up to a Knightmare. ( All Hail Armored Zero!! XD ) Lelouch is pretty cool in the Manga....that I assure you.
Contractor : C.C.
Mao :
As in Anime :
Mao's Geass proves to be a powerful weapon indeed. His Geass is not affected by direct eye contact and his range of efficiency is a radius of 500 metres. However , whether he can hear the minds of those around him or not depends on his concentration. He also is unable to switch off his Geass and he constantly hears the chatters of the minds around him which results in his loss of sanity.
Contractor : C.C.
As in Manga :
This Mao , is very different. In fact , there is even a gender change. Interestingly , her Geass is mind control and reading , forcing the victim to relieve their past. ( Something like Refrain , which it was named after )
Contractor : C.C.
Rolo Lamperouge :
As in the Anime :
Rolo Lamperouge's Geass is to stop the time in a person , but not his surroundings. Depending on his concentration , he can Geass upto a frightening range. However , the limitations of his Geass is that he can only stop the time within a person , that it only lasts for about 10 seconds and that the Geass will tire his body out if used constantly.
Contractor : V.V
Charles zi Brittania :
It's the big guy over there...Either way...
As in Anime :
Charles zi Brittania posses a disturbing Geass that can rewrite the memories of his victims entirely. Forgetting about the past they cherish most and painting over a fake memory if he wants to....a truly dangerous Geass to some.
Contractor : V.V ( Supposedly )
C.C. :
Can't believe I forgot about her :P
As in Anime :
Well , for the sake of spoilers , I can't really say much. Just watch the Anime and you'll know!!
Code Geass : Lost Colors :
Rai :
( Sorry , can't find pictures for the following characters )
As in Game :
Rai , uses a Geass that is extremely similar to Lelouch's. However , it utilises the usage of sound , not light.
Contractor : V.V
Code Geass : Nightmare of Nunnally :
Alice :
As in Manga :
Alice uses a very interesting Geass indeed. If her Geass is activated , she can move at incredibly high speed.
Contractor : C.C.
Dalque :
As in Manga :
Her Geass utilises the enchancement of strength.
Contractor : C.C.
Lucretia :
As in Manga :
Interestingly , her Geass makes her a walking map. it can fully scan and the area and provide with a 3D image within her mind of the terrain...oh boy , if only Lelouch had her in the Anime. ( Unfortunately , she's not an ally XD )
Contractor : I'm tired of this...most of the Geass holders are contracted with C.C. either way.
Nemoand Nunnally :
Only a image of Nunnally..but Nemo , when it transformed , looks exactly like Nunnally. Except that her hair is blonde and she have a fairly nasty look on her face.
As in Manga :
Nunnally and Nemo can Summon a Knightmare Frame and Nemo can read into the future , thus , recognising enemy movements a split second before they actually starts to operate. When using the Knightmare Frame , they merge into one and pilot as one , a truly frightening foe on the battlefield.
Contractor : Nemo
Sancia :
As in Manga :
She is practically a walking database. She can scan her intended target with her Geass and find out every single little thing about them. ( Their jobs , rank etc. etc. )
Nintendo DS Game :
Castor rui Brittania :
As in Game :
His Geass allows him to communicate via the mind. Telepathy , or something along those lines.
Pollux rui Brittania :
As in Game :
His the twin bro of Castor by the way.
His Geass utilises the same ability as Lelouch's , the Power of the Kings...sheesh , this is fairly popular in CLAMP isn't it?
( Side note : C.C.'s role in the Manga is far more sinister compared to her original self in the Anime , this explains the amount of people that are contracted with her )