Krakatoa was a volcanic island in the Sundra Straits of the Indonesia archipelago in the south Pacific Ocean. It erupted in 1883.
Some time prior to his third incarnation, the Doctor witnessed the eruption. During this event, he also heard Primords scream for the first time. (TV: Inferno)
That same day, a sketch of the Ninth Doctor washed up on a beach on Sumatra. It eventually found its way into Clive Finch's collection of artefacts related to the Doctor. (TV: Rose)
The Torchwood Institute tracked the Doctor to Krakatoa after the eruption. (COMIC: The Time Machination)
The Tenth Doctor and Rose Tyler visited Indonesia, where they encountered the alien Chalderans. They were captured and hung upside down, but the Doctor used his sonic screwdriver to free them. They escaped in the TARDIS just as the volcano erupted, killing the Chalderans. (COMIC: Under the Volcano)
The eruption powered the device which sent Gillian Petra to her native time period. (PROSE: Erasing Sherlock)
The eruption also caused a Xylok to be freed; this Xylok became Mr Smith many years later. (TV: The Lost Boy)