The Empire of Brittania has invaded Japan using giant robot weapons called Knightmare Frames. Japan is now referred to as Area 11, and its people the 11s. A Brittanian who was living in Japan at the time, Lelouch, vowed to his Japanese friend Suzaku that he'd destroy Brittania. Years later, Lelouch is in high school, but regularly skips out of school to go play chess and gamble on himself.

One day, he stumbles on terrorists 11s who've stolen a military secret and is caught by a member of the Brittanian task force sent after them, who is Suzaku. As the rest of the squad arrives, Suzaku is shot for disobeying orders, while the military secret, a young girl, gives Lelouch the power to make anyone obey any order. He takes command of the terrorists and leads them to crush the approaching Brittanian force. However, Suzaku is secretly made the pilot of Brittania's brand new test model Knightmare, Lancelot. Lelouch becomes the masked king Zero to lead the rebellion to defeat Brittania.

Note: Storyline is a bit different than the anime.

The Empire of Brittania has invaded Japan using giant robot weapons called Knightmare Frames. Japan is now referred to as Area 11, and its people the 11s.

A Brittanian who was living in Japan at the time, Lelouch, vowed to his Japanese friend Suzaku that he'd destroy Brittania. Years later, Lelouch is in high school, but regularly skips out of school to go play chess and gamble on himself. One day, he stumbles on terrorists 11s who've stolen a military secret and is caught by a member of the Brittanian task force sent after them, who is Suzaku. As the rest of the squad arrives, Suzaku is shot for disobeying orders, while the military secret, a young girl, gives Lelouch the power to make anyone obey any order. He takes command of the terrorists and leads them to crush the approaching Brittanian force.

However, Suzaku is secretly made the pilot of Brittania's brand new test model Knightmare, Lancelot. Lelouch becomes the masked king Zero to lead the rebellion to defeat Brittania.

This series follows Suzaku's side of the story, while Lelouch of the Rebellion follow's Lelouch's.

An alternate Code Geass story in which Lelouch's fate is unknown after his meeting with C.C., whether he be dead or alive. Nunnally makes a Geass contract and gains the power to read the lines of the future. She also obtains a new body and takes up piloting an unusual Knightmare Frame.

A thick mist covers the city of Faith-Delhi constantly. In the mist and smoke of this metropolis, demons known as "Pagans" lurk in the shadows. Pagans are born when humans fail to join the other souls in the path to reincarnation. There is a group known as the "Twanbach Funeral Service Company" who specializes in hunting these Pagans. Reginald, a 100+ year old vampire that looks like he's 20 years old, gets hired by the Twanbach company by the mysterious twins, Vanessa and Veronica Twanbach.

(Source: MangaHelpers)

4-koma parody of Code Geass featuring Lelouch Lamperouge as a part-time convenience store worker.

In a place called Polyphonica, humans and spirits live side by side. Young musicians create magical sounds with their music, which binds the spirits to them. A struggling student named Phoron accidentally summons a spirit he met in childhood but later forgot about. Phoron could use all the help he can get, but this feisty spirit may prove to be more than he can handle!
(Source: CMX)

A rather chibi version of the original Code Geass with a similar setting but in the Bakumatsu Era.