We know that Arjuna used the famous bow called Gandiva. This is an interesting story on how Arjuna came into possession of Gandiva bow. The Gandiva was made by Brahma and it could be used to One lakh enemies at a time. Later it came into possession of Indra, then to Chandra and then Varuna.
The whole story goes here.
There was a kingby the name Swetaki who was known for performing very long yagnas. His priests assisted him for a long time until they became frail. Once, Swetaki desired to conduct a one hundred years long yagna.
His priests, tired from the hardships done earlier, pretended to be angry and told the King to approach Rudra for help. Swetaki was disappointed but he anyway meditated upon Rudra for a while in the ranges of Kailash.
Finally, Lord Shiva appeased with Swetaki and agreed to help Swetaki under the condition that the King should first lead a life of Brahmachari for twelve years and he should continue to pour Ghee (clarified butter) into sacrificial fire without intermission for the whole period.
Swetaki did as advised, and a Lord Shiva appointed Durvasa as his priest who officiated the grand hundred-years long yagna. Swetaki, due to his virtuous nature, later ascended to the heavens.
This created a problem for Agni, who developed indigestion because of ghee feeding into fire for twelve years. Lord Brahma advised him to burn Khandava forest, this will cure his indigestion. Agni tried to burn the forest but failed as many as seven times but could not burn the Khandava forest.
Agni again asked Brahma for help, who advised him to approach Nara & Narayana, who were on earth in the form of Arjuna and Krishna.
Agni approached Arjuna and Krishna, who were living in the woods in the vicinity of the Yamuna. In exchange for their help, Agni requested for Varuna to give Krishna the mace by the name Kaumodaki and the Discus Chakrayudha that had a pole attached to it in the center. Varuna also offered Arjuna the bow Gandiva, two inexhaustible quivers and a chariot made by Viswakarma that had Hanuman in its banner. He also gave him white steeds that were born in the region of Gandharvas. This was the chariot in which Soma (Chandra) had earlier vanquished the Danavas.
This is how Arjuna received Gandiva and Krishna received Kaumodaki.