The user can animate corpses and dead beings, making them able to move and react. They are usually under the user's control, but because the corpses are likely mindless, they have to be carefully monitored. However, some users may be able to create sentient or semi-sentient undead.
Benny Weir (My Babysitter's a Vampire); using a reanimation potion
Marie Laveau (American Horror Story: Coven)
Morgana (The Sorcerer's Apprentice)
Herbert West (Herbert West the Re-Animator)
Imhotep (The Mummy)
An Shigeng (The Four)
Voldemort (Harry Potter)
Dark Wizards (Harry Potter)
Nico DiAngelo (Percy Jackson & The Olympians/The Heroes of Olympus)
No Life King (Grimgar of Fantasy and Ash); presumably
Hoodoo Mama (Wild Cards)
Sophie Mercer (Hex Hall Book 1/DemonGlass)
Vampires (True Blood/Sookie Stackhouse novels)
Menace (Queen's Blade series)
Phelous (Channel Awesome)
Phelous D-1 (Channel Awesome)
Black Lantern Spoony (Channel Awesome)
Santa Christ (Channel Awesome: Two-Year Anniversary special Kickassia)
Matoro (Bionicle); via his Kanohi Tryna
Jesus (Madness Combat)
Yatsufusa (Akame Ga Kill!)
Idol of Granath (Angel)
Kanohi Tryna (BIONICLE)
Ovu Mobani (Buffy the Vampire Slayer)
Element 115/Divinium (Call of Duty: Black Ops series)
Green Dip (Catscratch)
Resurrection Stone (Harry Potter)
Blacklight Virus (Prototype)
Herbert West’s Serum (Re-Animator)
T-Virus (Resident Evil Series)
Reanimation Potion (Sankarea)
Bob's Reanimation Machine (The Simpsons)
Orb of Rafuk (The Life and Times of Juniper Lee)
Yamamoto (Bleach) reanimating ten trillion skeletons of victims he killed.
Kagura (InuYasha) using her wind to utilize her Corpse Dance, controlling corpses as puppets.
Sō'unga (InuYasha) can reanimate entire army of corpses to serve as its minions and summon the souls of the dead...
including Setsuna Takemaru who So'unga chose as its host.
Orochimaru (Naruto) uses Summoning: Impure World Reincarnation to resurrects Hashirama Senju, the First Hokage, and Tobirama Senju, the Second Hokage.
Kabuto (Naruto) demonstrating the reanimating ritual.
Sohaku Kago (Tenjho Tenge) turned Fu Chi'en's corpse into a zombie, having him fight with his abilities intact.
Ovu Mobani (Buffy the Vampire Slayer) reanimates a nearby corpse.
Rea (Sankarea) holding a glass of the Reanimation Potion.
Stevie (Smosh) has the power to reanimate the dead with the power of love.
Jesus' (Madness Combat) most infamous power in the series is the ability to resurrect the dead. Up until Madness Depredation, he would often resurrect Hank's victims into zombies, in order to fight Hank. Although they do not possess better fighting skills, they are much more resistant to attacks than they are normally.
Element 115/Divinium (Call of Duty: Black Ops series) was the key material used by Group 935 to create several weapon programs, one of which was the undead army super soldier program for the Third Reich.
T-Virus/Tyrant Virus (Resident Evil)
The Red Marker (Dead Space) will reanimate and mutate anyone.