Some weapons about whom we all know:
Brahmastra: The weapon created by Brahma himself, Brahmastra comes the closest to any modern nuclear weapon in terms of destructive power and effects. Brahmastra finds its mention in Ramayana as well Mahabharat.
Ancient texts claim Brahmastra to be causing severe environmental damage, the land where the weapon was used became infertile, and so did the men and the women. Land developed cracks and life would cease to exist. Brahmastra is mostly used by "celestial figures" and one has to be mentally very strong and disciplined to be able to use it. Once fired, it could never be called back and would cause complete mayhem, much akin to the devastation caused by nuclear warfare.This weapon, in fact, could only be used once in a lifetime and was said to make land barren and prevent rainfall for upto 12 years.
Brahmashira: The most powerful weapon on the list. Also created by Brahma, this weapon was said to be four times as powerful as the Brahmastra, thereby literally dwarfing all the other weapons mentioned here. In fact using Brahmashira would've brought an end to the existence of everything and would've damaged the entire fabric of life beyond repair
Only two living beings had this weapon in their arsenal, one being the mighty warrior Arjun, who fought for the Pandavas in Mahabharata and the other being Ashwatthama , the son of the warrior teacher Drona.
Interestingly and perhaps luckily so, this weapon was never really used in war.
Pashupatashastra:On same lines as Brahmastra, this was the weapon primarily of Shiva's consort Mahakali, which literally means Goddess who is beyond time. The weapon finds its reference in Mahabharat when it was granted by Shiva to Arjuna and also in Ramayana when Indrajit fires it on Lakshmana. Devastating powers almost the same as Brahmastra, the only difference was that it was rarely granted as boon by Shiva, unlike the Brahmastra. This mighty arsenal of Shiva, this weapon was capable of ending all life on Earth, if the wielder chose to do so. One of the famous prerequisites for using this weapon was that it could not be used against lesser warriors and would not destroy the world, only if it was used against divine beings.Needless to say, this qualifies as a modern day nuclear weapon as well.
The only weapon that can neutralize Pashupatastra is the Kameswaraastra which is the personal weapon of Lord Kameswara, the consort of Adi Para Sakthi
Varunastra:The weapon created by Lord Varuna, the Lord of all oceans.
In a report published in 2008, several nations like China and U.S.A, plan to deploy weather as a potential weapon, as per which they plan to seed clouds with chemicals like ammonium nitrate, increasing their density and then a jet of rain-forming chemicals is fired, creating artificial rains, which are chemically lethal too. The artificial rains could also be used to restrict the movement or devastate the conditions of the opposition army.
On similar terms, when Varunastra was used, it would emit smoke, which would convert into clouds and then rain, as soon as it gets in contact with air. So, Mahabharat already talks about weather warfare.
Vayavastra: Created by Lord Vayu, the Lord of winds, this weapon, as per Mahabharat, when fired would create artificial winds. Though used primarily to counter the above mentioned Varunastra, it would disperse clouds formed by Varunastra.
This again finds parallels with weather modification warfare as mentioned previously. Huge winds are created artificially to restrict movements of opposition.
Vajra: The thunderbolt weapon of Indra, much akin to the thunderbolt weapons of Gods of other beliefs, like Jupiter or Zeus. Vajra' or the divine Lightning Bolt, which he used occasionally to smite his enemies , could not only cut through mortal defences but was equally capable of destroying multiple divine forces.
Gada: While the Gada of Hanuman finds close resemblance to Thor's hammer.
Shiva's Bow: So basically Shiva had a lot a weapons which he used for destroying things, since his role in the holy trinity was that of a destroyer. The bow measured a whopping 8.5ft in length and was passed on by Shiva to a few deserving mortal men, who more often than not were reincarnations of celestial beings.
This weapon, however, was eventually broken, by another god incarnate known as Rama, who had been given the task of stringing the bow, in order to win the hand of his lady love Sita.
Shiva's Trishula : The ultimate weapon of mass destruction and annihilation in Hindu Mythology is Lord Shankar's three-pronged spear or the Trishula. It is known that the three prongs of Trishula represent the powers of will, knowledge and action. It is the supreme weapon that also portrays the three aspects of nature viz. creation, sustenance and dissolution. The power of this destroy all evil and ignorance in this world. No mean feat, eh?
Over the course of history, Shiva is believed to have used this weapon to kill numerous demons who could not be destroyed by lesser beings and warriors.
It is believed that a stabilizer would facilitate its flight when thrown towards an enemy.
Agni Astra (Fire weapon) : The name pretty much says it all. Wielded by numerous warriors, this divine weapon was basically the ancient equivalent of a Flamethrower. Various depictions show the weapon to be in the form of an arrow that when launched emits fire from its pointy end.
Nagapash (Snake weapon) : As the name suggests, this weapon was used to literally entwine the enemy in a thousand venomous snakes. It was famously used against the mythical hero Rama and his brother Laxmana by Ravana's brother, Indrajit- named so after his victory over the Head of Devas, Indra.
In fact very few warriors could lay claim to possessing this weapon and more often than not, those warriors were not fighting for the good side!!