Shesha or Ananta Nag is the massive thousand headed serpent who floats on the cosmic ocean, serving as the resting bed of Sri Vishnu. He holds the universe on his hoods and sings the glories of the Lord.
‘Shesha’ means what remains when the universe dissolves while ‘Ananta’ means endless. Krishna extols Shesha in the Gita, “Of the Nagas I am Ananta” – ‘Naga’ refers to many hooded serpents in contrast to ‘sarpa’ which is a normal snake.
Varaha (boar) avatar dived into the cosmic waters to save Prithvi (earth). Shesha supported the boar’s feet while rising from the water.
The image from masalabox.co.in shows a sculpture at the Gupta era Udaygiri caves in MP, clearly depicting Shesha supporting Varaha.

Shesha is associated with Vishnu, Vasuki with Shiva. Krishna in the Gita says that He is Vasuki among sarpas (snakes).
In the ‘Sagar Manthan’ episode, Vasuki is used as the rope to churn the cosmic ocean using Meru mountain. Before amrit (nectar), it is Vasuki’s poison that emerges and is swallowed by Shiva and lodged in His throat making it blue, hence His name Nilakantha. When setting ground for a new house, Vasuki is worshipped as the foundation. Folklore suggests that the foundation of the iron pillar in Delhi was dug up by the Tomar king and blood gushed out as Vasuki was hurt. The Hindus lost Delhi soon after.
Vasuki is worshipped as Basaki in the Himalayas even today. Indonesia still has a town named Besaki named after the cosmic snake.
The image by S. R. Ramanujam is of the Mrikula Devi temple in Himachal which has a depiction of Vasuki’s role in Sagar Manthan.

Takshaka was a powerful Naga king, son of Kashyap and Kadru.
When Arjuna & Krishna burnt the Khandavaprastha to create Indraprastha, Takshaka had to flee to save his life. Takshaka’s son sat on Karna’s arrow that almost killed Arjuna in the war. Arjuna was saved Krishna sank the chariot into the ground with his toe. The arrow hit Arjuna’s crown and not head.
Arjuna’s grandson and successor of Pandavas was Parikshit. He was bitten to death by Takshaka following a rishi’s curse. Parikshit’s son Janmajeya conducted the sarpa-satra (snake sacrifice) as a revenge. Mahabharata was first recited during this Yajna. Astika saved the snakes from being wiped off.
Takshaka’s poison is a metaphor for grave danger. Susruta the great ayurveda doctor calls his antidote to poison as ‘Tarkshya’.
The image from pilgrimaide.com shows the Takhaji temple at Naole near Indore where Takshaka is worshipped alongwith Dhanwantri, god of healing.

Kaliya took refuge in Yamuna or Kalindi near Brindavan after a dispute with Garuda. Kaliya’s deadly poison dried up the trees on the bank and birds flying overhead dropped dead.
Once Krishna & friends were grazing their cows close to the place. Feeling thirsty, the cowherds drank the dark water and died. Krishna revived them and fought fiercely to subdue Kaliya after which he danced on his multiple hoods.
Harivamsa (annex to Mahabharata) tells a straightforward story but the Bhagvatam imbues it with bhakti – the poisoned river is our mind, the snake’s multiple hoods are desires and Krishna is redemption.
The image from imdb shows the 1919 film ‘Kaliyamardan’ , one of the first films made in India by Dadasaheb Phalke.

Lalitaditya Muktapida (724-60) reigned from Kashmir and ruled over an area from Central Asia to Ganga valley. Kalhana’s Rajatarangini says that Kashmir was a centre of snake worship. Nagas were water spirits who took refuge from Garuda in Kashmir and were guardians of lakes and springs - Nilamata Purana names 527 such snakes. They granted timely rain when worshipped properly; and when angry they caused hailstorms and floods.
In Kashmiri, the word nag means the source of a river e.g. Anantnag is source of mighty Vitasta (Jhelum). At Kashyapa’s request, Balarama drained the valley of water demons using his ploughshare ( some say Kashmir comes from Kashyapa’s mir – lake), making it fit for human habitation. The water demon tamed by Balarama is Vernag.
The Wikimedia image by Bhatwasi shows the remnants of Lalitaditya’s Naranag temple at Ganderbal.

A peculiar story of Manasa Devi used to be popular in the Hooghly region of Bengal. A merchant refused to propitiate the powerful snake goddess. In anger, she asked her snake children to bite and kill the merchant’s family. Just a son survived but still the merchant was unmoved. When the son married, the snakes bit him too. The daughter-in-law persuaded Manasa Devi to return her husband.
The merchant then realized his folly and agreed to worship the goddess. But since he was angry with the goddess’ past actions, he used his left hand for worship. The use of left-hand worship prompted academics to suggest that Manasa Devi was a folk deity from outside the mainstream Hindu tradition. But Manasa Devi temples exist across the country and the one at Haridwar is a siddha peetha (wish fulfilling). The image from incredibleindia.gov.in shows the Manasa Devi temple at Haridwar.

Shesha’s avatars as Lakshmana in Ramayana & Balarama in Mahabharata are popular. In Ramayana, Shesha incarnates as Rama’s younger brother. In Mahabharata he appears as Bala Rama. A mighty white snake comes out of Balarama’s mouth when he dies. Ancient Naga images from the Mathura region are locally referred to as ‘Tau ji’ since Balarama was the elder brother of Krishna avatar. On Naga Panchami, in many places farmers don’t plough so that they don’t hurt snakes on their day. Balarama is also closely associated with the plough.
The Wikimedia image by Biswaroop Ganguly shows the 2nd cent CE Kushan era Naga sculpture from Chargaon, Mathura. Notice the likeness to Balarama and the snake hood.
Even historical figures attribute their descent to Nagas. In Varanasi, there is a naga kuan (well) dedicated to Shesha’s avatar as Patanjali, the writer of Mahabhashya – the famous commentary on Panini. On Naga Panchami, grammarians gather here for a discussion.

Legends say that Parasurama threw his axe into the sea to create Kerala from the sea but the land was too salty. He prayed to Shiva who asked him to request Nagaraja to help him. Nagaraja used his poison to neutralize salinity. In gratitude, Parasurama gave the snakes the land in Vetticode to move around freely. The Addimoolam (first) Sri Nagaraja temple came up dedicated to snakes.
Most households in early days had a small kavu (sacred grove) for snakes where a small icon was worshipped. Over the years, many of these icons were moved to the Vetticode temple which now has more than 100 such murtis.
The Wikimapia image shows snake icons at the Vetticode Aadimoolam temple.
Source: Sahapedia

Mannarasala has a centuries-old unique system of worship in which the eldest female becomes the chief priestess; the priestess is succeeded by the second eldest female member of the family. The high priestess is called amma (mother); when a woman is appointed as the next amma, she takes a vow of celibacy and adopts an ascetic lifestyle for the rest of her life.
According to legends, the place where Parasuram did penance to please Nagaraja became the temple of Mannarasala. The place was surrounded by mandara (ethyrina) trees & many medicinal plants. When Nagaraja appeared before Parasuram, he agreed to reside there and eternally bless the land with his presence. Thus, the abode of Nagaraja among the mandara trees came to be known as mandarasala, later Mannarasala. The temple has been maintained by the Mannarasala Illam (household).
The picture by S. R Ramanujam is of Mannarsala temple.

The Nag river flows through Nagpur. The river takes a serpentine shape and that is supposed to be the reason it is called Nag river.
The city municipal corporation’s logo has an image of a Cobra in a river.
Nagaur, Nagda and many other city names are linked to snake tradition.

The Nagaraja temple (9th-12th cent) at Nagercoil (Nager-Naga, Koil-temple) gives the name to the city. The sanctum floor of the temple is uniquely made of sand and houses the five headed Nagaraja. Wet sand is given as sacred prasadam.
South India has a living tradition of snake worship. The Vaidya community are skilled in playing the small drum ‘dakke,’ and are considered masters of snake lore. In South India snake slabs (Naga kals) are a common sight in temples and near snake hills. Often a childless woman makes a vow to install one if blessed with a child.
Many such snake stones from Vijayanagara times have been found. The Adichallanur naga kals are unique as they are three dimensional.
18th cent onwards, Naga worship got associated with Subramanya and today Naga worship invariably happens near Subramanya temples. The Kukke Subhramanya temple in Karnataka is a famous temple where Subhramanya is worshipped as the ruler of snakes.
The picture by S. R. Ramanujam shows worship of Naga murtis outside the Nagercoil temple.

Himachal has many ancient Naga temples, mostly around groves of deodar trees and near village water bodies. The 10th cent Khajji Nag temple on a meadow, called Switzerland in India, is perhaps the oldest. Here Nagaraja is worshipped along with Shiva & Hidimba Devi, the rakshasi wife of Bheema.
In Himachal, Naga & Devi worship intertwine. The simple hut shaped shrines of wood & stone are the earliest style of temple architecture in the Himalayas. Snake worship is a living tradition here. After Diwali in Kangra, snake idols made of cowdung are worshipped and bid goodbye. If a snake is seen after this, it is considered ‘ungrateful’ and killed. The image is of the Khajji Nag temple at Khajjiar from TripAdvisor.