The Cortana Letters
In early 1999, Hamish Sinclair, the famed Story Guy for the Marathon's Story Page site, began receiving rather cryptic messages from an entity calling itself "Cortana". This was before Halo had even been revealed at MacWorld New York 1999, though rumors of a new Bungie game were flying about rampantly, as it were. The Cortana Letters can be found here:
The Cortana we are shown is of a temper we have seen before elsewhere; hungry and vengeful (note the sloping, criminal forehead). While not neccessarily canon material, these letters do provide exceptional insights into the development (and future?) of not only Cortana, but Halo's story as a whole. Please feel free to speculate about the letters both here on the Halo Story Page and on the Marathon Story Page as well.