Hunter X Hunter - Greed Island
The story takes place after the first OVA.
After the conflict with the spiders, Gon and Killua return to the York Shin auction to try their luck with Greed Island. Once inside, Gon and Killua meet up with Biscuit, Wing's old teacher, who helps them train to defeat the monsters and player killers found inside the game.
(taken from Animenfo)
Info: OVA, 8 episodes, on air from 05.02.2003 till 16.04.2003