For information on the "Cannibalize" racial ability, see Cannibalize
Most trolls seem to practice cannibalism. Several specific troll tribes have been confirmed as cannibalistic, including the Mossflayer and Vilebranch tribes. Furthermore, all of the currently known tribes of ice trolls practice cannibalism. Sand trolls, forest trolls, and jungle trolls are also frequently cannibalistic, though not always. Even the mysterious dark trolls are believed to be cannibals for the most part. Yet there are several notable exceptions. For example, the Zandalar and Revantusk tribes do not consume troll flesh. The Darkspear tribe, too, was cannibalistic until it joined the Horde, at which point the Darkspears officially gave up cannibalism. 
- The Darkspear trolls come from a dark and bloodthirsty history of sacrifice, cannibalism and black magic. They consider spirits to be individuals much like living creatures. Spirits are greedy, hostile and dangerous. Trolls also believe their ancestors linger on as jealous spirits who miss the land of the living and require blood sacrifices to appease them.
- Trolls sacrifice and eat their enemies. They conduct these practices for two reasons. First, they believe the sacrifice of sentient creatures appeases malicious spirits. Second, they believe that after death, an enemy’s spirit can visit misfortune on its killer. By consuming the flesh of their enemies, trolls believe they can also consume their enemy’s spirit, or at least damage it enough to render it impotent.
- The orcs’ influence tempers the Darkspear trolls’ spiritual beliefs. The trolls willingly support Thrall and the Horde, and they understand that their destructive rituals offend their allies. Under Thrall’s tutelage, the Darkspear trolls abandoned the sacrifice of sentient creatures and took up animal sacrifice instead. These trolls no longer eat their enemies, but practice other methods of trapping, injuring or destroying enemy spirits. These methods include witch doctor blessings, the burning of enemy hearts, drowning corpses and head-shrinking.
- Most members of the Horde look askance at the practice of head-shrinking, but consider it a step up from human sacrifice and cannibalism. Some trolls have techniques to shrink skulls as well, which involve removing key pieces and reconstructing the skull as a smaller version using animal parts and resins to hold it together.
- In most tribes, jungle trolls regularly practice cannibalism. Jungle trolls believe that by eating the flesh of their enemies, they not only appease the spirit of the deceased but also consume a portion of that spirit. Thus, by cannibalizing fallen foes, jungle trolls make sure that the mischievous spirits of their enemies do not visit misfortune upon them. They thus have no qualms about devouring an enemy defeated in combat, be he troll or not. Though until recently the Darkspear trolls adhered to these ancient beliefs, their interaction with the Horde has taught them restraint and other virtues. Other beliefs, especially those of the orcs, also “pollute” their ancient traditions...The Darkspear tribe no longer practices cannibalism — at least not openly. Some trolls revert to their old ways out of misunderstanding, and some refuse to adapt, but most accept the Horde’s ways as their own.
- Darkspear tribe are an entirely different group; before the Third War, they lived in the Broken Isles, on an island near the Maelstrom, practicing their ancient cannibalistic variety of voodoo and tainted shamanism.
- In contrast to their wily jungle troll cousins, forest trolls are savage and unrelenting creatures. Not only are they cannibals like other trolls, forest trolls live for slaughtering lesser races, especially the high elves, whom they despise.
It seems that forest trolls practice cannibalism more for the meal than the magic, seeing it as more of a food source than a method to tame spirits. It should be noted that not all troll tribes engage in this practice. The Revantusk trolls, for example, describe the cannibalism of their enemies, the Vilebranch, as "depraved."
Bonechewer Orc Clan
In Beyond the Dark Portal, a small band of orcs, led by Fenris Wolfbrother, enter Menethil Harbor secretly to steal boats from the Alliance and sail to the raised Tomb of Sargeras. Fenris is initially skeptical of the orcs ability for stealth, though he is relieved somewhat by their initial success at the venture. He is quickly disappointed when the alarm is sounded by the humans, who were alerted by a guard's wails from being eaten alive by a Bonechewer orc, in the middle of the operation.
Desperate broken will sometimes resort to cannibalism. 
Savage wendigo have been known to be cannibalistic. Its implied that they eat their own species.
Webster defines Cannibalism as;
- Main Entry: can·ni·bal·ism
- Pronunciation: \ˈka-nə-bə-ˌli-zəm\
- Function: noun
- Date: 1796
- 1 : the usually ritualistic eating of human flesh by a human being
- 2 : the eating of the flesh of an animal by another animal of the same kind
In World of Warcraft cannibalism appears to be the practice of a sapient humanoid species eating another sapient humanoid species, whether one of their own or one of another.
Other possible cannibalism?
While described as cannibals by some sources, or seemingly meeting the criteria of cannibalism as described regarding the trolls, the status of these races as cannibals is unconfirmed.
Ogres may or may not also habitually eat murlocs; Mudcrush Durtfeet tasks adventurers with bringing him murloc heads to eat.  Ogres have also been known to eat orcs, and presumably other humanoid races they've had conflict with.
Centaur have also been known to practice cannibalism; some of them once captured Baine Bloodhoof and planned to eat him. Also, the Mauradine centaur in Desolace have captured Melizza Brimbuzzle, intending to make a meal of her.
The carnivorous gnolls consider all creatures potential food, including members of their own race. "More bones to gnaw on" is commonly said by gnolls when they encounter another humanoid race.
The massive magnataur of northrend are constantly hungry due to their ever increasing size, and when raiding human caravans will eat the humans, horses, and anything they can find that won't break their teeth.