The Forerunner

Summary
The "Forerunners" is the translated Covenant name for the race that built The Halos. The Forerunners were the creators of the 7 Halos and the many facilities that lie buried and forgotten in countless worlds scattered around the galaxy. Their technology is superior to both Human and Covenant, and it is commonly theorized that they created the Humans, the Covenant, and the Flood, but for reasons unknown.

When the Forerunners encountered (or created?) the Flood, they went to war, trying desperately to defeat the parasitic species. When the Forerunners ran out of defensive options, they remotely detonated the Halos from the installation called The Ark, theorized to be on Earth and may be featured in the Halo 3 Announcement Trailer.It is theorized that this same race is responsible for the artifact that was found on Reach, and on Sigma Octanus IV.

The artifacts appear to have originated on different worlds, but have somehow made it onto these other planets. These artifacts are our only means of finding information about the Forerunner.

In Halo 2, 343 Guilty Spark explains that after exhausting every other strategic option, the Forerunners destroyed themselves along with other sentient life three radii of the galactic center by activating the Halos.

Maybe they survived the explosion and evolved into us. Maybe they didn't. There is still much to learn about the Forerunners, and rumors are still flying.

Hopefully all will be answered in Halo 3.