The Dead series

Romero's Dead series:

1. Night of the Living Dead (1968)
2. Dawn of the Dead (1978)
3. Day of the Dead (1985)
4. Land of the Dead (2005)
5. Diary of the Dead (2007)
6. Survival of the Dead (2009)

Remakes:

Night of the Living Dead (1990)
Dawn of the Dead (2004)
Night of the Living Dead 3D (2006) [direct to video]
Day of the Dead (2008) [direct to video]

Living Dead series:

1. Night of the Living Dead (1968)
2. Return of the Living Dead (1985)
3. Return of the Living Dead Part II (1988)
4. Return of the Living Dead III (1993)
5. Return of the Living Dead: Necropolis (2005) [direct to cable]
6. Return of the Living Dead: Rave to the Grave (2005) [direct to cable]

Unofficial sequels:

Zombi 2 (1979)
Zombi 3 (1988)
Zombi 4: After Dead (1989)
Zombie 5: Killing Birds (1988)
Children of the Living Dead (2001) [direct to video]
Day of the Dead 2: Contagium (2005) [direct to video]
Flight of the Living Dead: Outbreak on a Plane (2007) [direct to video]
Night of the Living Dead 3D: Re-Animation (2012)
Night of the Living Dead: Origins 3D (2013)