Heimdall's Observatory

The Bifrost (Rainbow Bridge) is the bridge which connects Asgard to the rest of the Nine Realms. It was destroyed during Thor's confrontation with Loki.

It is guarded and used by Heimdall

Thor
It was used to transport Thor and his friends to Jotunheim, Odin then used it to rescue them from from Jotunheim.

After the battle Odin used it to banish Thor from Asgard.

During Thor's confrontation with Loki, it was destroyed in order to prevent the obliteration of Jotunheim

Power Source
The bridge is activated by Heimdall's Sword, or by Gungnir, Odin's Staff.

It is hinted that the Tesseract has the ability to power the bridge

Appearances
Thor - (first appearance) (destruction)