The following is a list of Mesopotamian deities.

Major Deities  
Hadad (or Adad) - storm and rain god
Enlil (or Ashur) - god of air, head of the Assyrian and Sumerian pantheon
Anu (or An) -  god of heaven and the sky, lord of constellations, and father of the gods
Dagon (or Dagan) - god of fertility
Enki (or Ea) - god of the Abzu, crafts, water, intelligence, mischief and creation and divine ruler of the Earth and its humans
Ereshkigal - goddess of Irkalla, the Underworld
Inanna (later known as Ishtar)  - goddess of fertility, love, and war
Marduk - patron deity of Babylon who eventually became regarded as the head of the Babylonian pantheon
Nabu - god of wisdom and writing
Nanshe - goddess of prophecy, fertility and fish
Nergal - god of plague, war, and the sun in its destructive capacity; later husband of Ereshkigal
Ninhursag (or Mami, Belet-Ili, Ki, Ninmah, Nintu, or Aruru) - earth and mother goddess
Ninlil - goddess of the air; consort of Enlil
Ninurta - champion of the gods, the epitome of youthful vigor, and god of agriculture
Shamash (or Utu) - god of the sun, arbiter of justice and patron of travelers
Sin (or Nanna) - god of the moon
Tammuz (or Dumuzid) - god of food and vegetation

Minor deities
This is only some of them. There are thousands. This is in Alphabetic order.
Abu - a minor god of vegetation
Ama-arhus - Akkadian fertility goddess; later merged into Ninhursag
Amasagnul - Akkadian fertility goddess
Amurru - god of the Amorite people
An - a goddess, possibly the female principle of Anu
Arah - the goddess of fate.
Asaruludu or Namshub - a protective deity
Ashnan - goddess of grain 
Aya - a mother goddess and consort of Shamash
Azimua - a minor Sumerian goddess
Bau - dog-headed patron goddess of Lagash
Belet-Seri - recorder of the dead entering the underworld
Birdu - an underworld god; consort of Manungal and later syncretized with Nergal
Bunene - divine charioteer of Shamash
Damgalnuna - mother of Marduk
Damu - god of vegetation and rebirth; possibly a local offshoot of Dumuzi
Emesh - god of vegetation, created to take responsibility on earth for woods, fields, sheep folds, and stables
Enbilulu - god of rivers, canals, irrigation and farming
Endursaga - a herald god
Enkimdu - god of farming, canals and ditches
Enmesarra - an underworld god of the law, equated with Nergal
Ennugi - attendant and throne-bearer of Enlil
Enshag - a minor deity born to relieve the illness of Enki
Enten - god of vegetation, created to take responsibility on earth for the fertility of ewes, goats, cows, donkeys, birds
Erra - Akkadian god of mayhem and pestilence
Gaga - a minor deity featured in the Enûma Eliš
Gatumdag - a fertility goddess and tutelary mother goddess of Lagash
Geshtinanna - Sumerian goddess of wine and cold seasons, sister to man 
Geshtu-E - minor god of intelligence
Gibil or Gerra - god of fire
Gugalanna - the Great Bull of Heaven, the constellation Taurus and the first husband of Ereshkigal
Gunara - a minor god of uncertain status
Hahanu - a minor god of uncertain status
Hani - an attendant of the storm god Adad
Hayasum - a minor god of uncertain status
Hegir-Nuna - a daughter of the goddess Bau
Hendursaga -  god of law
Ilabrat - attendant and minister of state to Anu
Ishum - brother of Shamash and attendant of Erra
Isimud - two-faced messenger of Enki
Ištaran - god of the city of Der (Sumer)
Kabta - obscure god “Lofty one of heaven”
Kakka - attendant and minister of state to both Anu and Anshar
Kingu - consort of Tiamat; killed by Marduk, who used his blood to create mankind
Kubaba - tutelary goddess of the city of Carchemish
Kulla - god of bricks and building
Kus (god) - god of herdsmen
Lahar - god of cattle
Lugal-Irra - possibly a minor variation of Erra
Lulal - the younger son of Inanna; patron god of Bad-tibira
Mamitu - Sumerian goddess of fate
Manungal - an underworld goddess; consort of Birdu
Mandanu -god of divine judgment
Muati - obscure Sumerian god who became syncretized with Nabu
Mushdamma - god of buildings and foundations
Nammu - a creation goddess
Nanaya - goddess personifying voluptuousness and sensuality
Nazi - a minor deity born to relieve the illness of Enki
Negun - a minor goddess of uncertain status
Neti - a minor underworld god; the chief gatekeeper of the netherworld and the servant of Ereshkigal
Nibhaz - god of the Avim
Nidaba - goddess of writing, learning and the harvest
Namtar - minister of Ereshkigal
Nin-Ildu - god of carpenters
Nin-imma - goddess of the female sex organs
Ninazu - god of the underworld and healing
Nindub - god associated with the city Lagash
Ningal - goddess of reeds and consort of Nanna (Sin)
Ningikuga - goddess of reeds and marshes
Ningirama - god of magic and protector against snakes
Ningishzida - god of the vegetation and underworld
Ninkarnunna - god of barbers
Ninkasi - goddess of beer
Ninkilim - "Lord Rodent" god of vermin
Ninkurra - minor mother goddess
Ninmena - Sumerian mother goddess who became syncretized with Ninhursag
Ninsar - goddess of plants
Ninshubur - Sumerian messenger goddess and second-in-command to Inanna, later adapted by the Akkadians as the male god Papsukkal
Ninsun - "Lady Wild Cow"; mother of Gilgamesh
Ninsutu - a minor deity born to relieve the illness of Enki
Nintinugga - Babylonian goddess of healing
Nintulla - a minor deity born to relieve the illness of Enki
Nu Mus Da - patron god of the lost city of Kazallu
Nunbarsegunu - goddess of barley
Nusku - god of light and fire
Pabilsaĝ - tutelary god of the city of Isin
Pap-nigin-gara - Akkadian and Babylonian god of war, syncretized with Ninurta
Pazuzu - son of Hanbi, and king of the demons of the wind
Sarpanit - mother goddess and consort of Marduk
The Sebitti - a group of minor war gods
Shakka - patron god of herdsmen
Shala - goddess of war and grain
Shara - minor god of war and a son of Inanna
Sharra Itu - Sumerian fertility goddess
Shul-pa-e - astral and fertility god associated with the planet Jupiter
Shul-utula - personal deity to Entemena, king of the city of Eninnu
Shullat - minor god and attendant of Shamash
Shulmanu - god of the underworld, fertility and war
Shulsaga - astral goddess
Sirara - goddess of the Persian Gulf
Siris - god of beer
Sirsir - god of mariners and boatmen
Sirtir - goddess of sheep
Sumugan - god of the river plains
Tashmetum - consort of Nabu
Tishpak - tutelary god of the city of Eshnunna
Tutu - tutelary god of the city of Borsippa
Ua-Ildak - goddess responsible for pastures and poplar trees
Ukur - a god of the underworld
Uttu - goddess of weaving and clothing
Wer - a storm god linked to Adad
Zaqar - messenger of Sin who relays communication through dreams and nightmares

Primordial beings
Abzu - the Ocean Below, the name for fresh water from underground aquifers; depicted as a deity only in the Babylonian creation epic Enûma Eliš
Anshar - god of the sky and male principle
Kishar - goddess of the earth and female principle
Kur - the first dragon, born of Abzu and Ma.  Also Kur-gal, or Ki-gal the underworld
Lahamu - first-born daughter of Abzu and Tiamat
Lahmu - first-born son of Abzu and Tiamat; a protective and beneficent deity
Ma -primordial goddess of the earth
Mummu - god of crafts and technical skill
Tiamat - primordial goddess of the ocean

Demigods and Heroes
Adapa - a hero who unknowingly refused the gift of immortality
The Apkallu - seven demigods created by the god Enki to give civilization to mankind
Gilgamesh - hero and king of Uruk; central character in the Epic of Gilgamesh
Enkidu - hero and companion of Gilgamesh
Enmerkar - the legendary builder of the city of Uruk
Lugalbanda - second king of Uruk, who ruled for 1,200 years
Utnapishtim - hero who survived a great flood and was granted immortality; character in the Epic of Gilgamesh

Spirits and demons
Alû, demon of night
Asag - monstrous demon whose presence makes fish boil alive in the rivers
Asakku, evil demon(s)
The edimmu - ghosts of those who were not buried properly
Gallû, underworld demon
Hanbi or Hanpa - father of Pazuzu
Humbaba - guardian of the Cedar Forest
Lamashtu - a malevolent being who menaced women during childbirth
Lilû, wandering demon
Mukīl rēš lemutti demon of headaches
Pazuzu - king of the demons of the wind; he also represented the southwestern wind, the bearer of storms and drought
Rabisu - an evil vampiric spirit
Šulak the bathroom demon, “lurker” in the bathroom
Zu - divine storm-bird and the personification of the southern wind and the thunder clouds

Legendary beasts
Battle Bison beast - one of the creatures slain by Ninurta

The eleven mythical monsters created by Tiāmat in the Epic of Creation, Enûma Eliš:

 Bašmu, “Venomous Snake”
 Ušumgallu, “Great Dragon”
 Mušmaḫḫū, “Exalted Serpent”
 Mušḫuššu, “Furious Snake”
 Laḫmu, the “Hairy One”
 Ugallu, the “Big Weather-Beast”
 Uridimmu, “Mad Lion”
 Girtablullû, “Scorpion-Man”
 Umū dabrūtu, “Violent Storms”
 Kulullû, “Fish-Man”
 Kusarikku, “Bull-Man”

Mesopotamian