In the Halo Universe there are several references to Norse Mythology, or simply the Norse people. Below are a few of them.
Loki; Loki (Also known as Mack) was an AI on Harvest who could shift shape. Loki, in Norse Mythology, is the God of chaos, mischief and treachery, and could shift shape like Mack. Additionally, he was also infamously known to be a trickster - much like Mack.
Sif: Sif was another A.I. on Harvest, and was often annoyed by Loki. In Norse Mythology, Sif is the wife of Thor and the Goddess of wheat and fertility. Ironically, Loki stole her hair in a myth.
Jotun: Jotuns were machines used on Harvest. Mythologically, they were giants.
Fenris: Fenris warheads were used by the Spartans at the end of Ghosts of Onyx. Fenris or Fenrir is a giant wolf from Norse myth, who was also the son of Loki.
Utgard: Utgard was the capital of Harvest. In Norse mythology, Utgard was the stronghold of the giants.
Bifrost: Bifrost was a bridge that connected Midgard and Asgard.
Valhalla: Valhalla is a map in Halo 3. In Norse mythology, Valhalla is the Hall of Odin, a place where fallen warriors who died in combat went to battle all day and feast all night forever. Each time a warrior was killed, he would simply respawn, similar to the process of Multiplayer.
Vigrond: Vigrond Highlands was a place near Utgard on Harvest. In Norse mythology, it is the plains where Ragnarok is said to take place.
Ida: Ida is named after the plains in Asgard.
Yggdrasil: The HRUNTING/YGGDRASIL Mark I Prototype Armor is named after Yggdrasil the World Tree.
The Index: The Activation Index is similar to the traditional symbol of Thor's Hammer.
Gungnir Armor: The Mjolnir Powered Assault Armor/(G) variant, or GUNGNIR armor is named after the Gungnir, as mentioned above.
Skidbladnir: The UNSC Skidbladnir is named after the ship made by Loki to fit all the Gods. It was also small enough to fit into a pocket.
MJOLNIR: The Mjolnir Powered Assault Armor, that is designed specifically for the SPARTAN-II's, is also the hammer of Thor. Mjolnir translates simply to "crusher", referring to the hammer's pulverizing effect. The hammer also shot bolts of lightning, since Thor was the Norse God of lightning; and when thrown, the hammer would return to the owner's hand.
Berserk: Sometimes a Brute goes on a rampage nicknamed "berserking". The Norse had warriors, called Berserkers who were followers of the cult of Odin (the one-eyed Norse god of war) and believed in the importance of a warrior dying heroically in battle rather than shamefully in his bed. They were said to have been immune to pain and possess uncontrollable aggression, and fought wearing only bear or wolf skins, entering a trance-like state before battle. When berserkers went into their battle state of mind, they tended to lose their sense of reason, mercilessly attacking and killing anything in the way of their weapons. The berserker's behavior seems to come from ingestion of drugs such as alcohol before combat, although that would make them difficult to use on the battlefield, similar to the Brutes from Halo 2 as well as Halo 3.
Grunt Symbol: The Grunt Symbol emblem in Halo 3 closely resembles the Slavic Kolovrat or "Sun Wheel," a symbol also used in Nordic Paganism for the eight directions and dimensions, and as representing the sun as well. However the Halo 3 emblem has only six spokes, where as the Kolovrat has eight.
Ragnarok: Ragnarok is a map in Halo 4. In Norse Mythology, Ragnarok is the battle of the gods, when many important Norse figures, including Thor and Loki, died, leaving only two humans to repopulate the world.