EDIT: Including anime episodes.
Movies:
In the movie, it lives in a temple in the mountains of Gracidea. It had been asleep for centuries, for so long that moss began to grow on its body. However, when Zero entered the Reverse World and began shattering pillars of ice within it, the glacier in front of Regigigas' temple suffered catastrophic explosions and began moving. This disturbed the great titan's slumber, rousing it into action. When Dawn's Buneary and Swinub failed to stop the glacier with the help of a large group of wild Pokémon, Regigigas appeared in the command of a herd of dozens of Mamoswine and together, they began pushing on the glacier to hold it back. Regigigas returned to its temple at the end of the movie, where it is seen using Hyper Beam on Team Rocket during the credits.
A Regigigas appeared in the opening scenes
Anime Episodes
A Regigigas appeared at the Snowpoint Temple. J wanted to steal it so it was rudely awakened by her. It went on a rampage after, using Hyper Beam on practically anything. It even used Confuse Ray to control Pyramid King Brandon's set of legendary golems to help it. At the end of the episode, it healed Brandon and his golems using Hidden Power to restore them from their petrification. It was then taken back to the temple to return to its slumber.
A Regigigas appeared as part of Meowth's boss fantasy alongside a Giratina
Source: http://bulbapedia.bulbagarden.net/wiki/Regigigas
