Spike-Quilled Axe - Successfully complete all Hunter secondary missions to unlock the Spike-Quilled Axe. Note: The Hunter is located near the large stone cross in the Grasslands, and he will move halfway through his missions to the Ice Barracks. Once you complete all Hunter secondary missions, return to the first place you met him, and he will give you the Steel-Quill Axe.

Extra nitrous power - Successfully complete all Racing secondary missions to get increased nitrous power.

Final interactions - Successfully complete the game. Then, go to one of the following locations to find the corresponding character(s) and have a final interaction with them:

Fire Baron: In the jungle directly north of the tree island at the bottom of the last continent. He is to the left of the main road as soon as it turns, traveling south to east.
Infantry/Army: At the Beach Party
Lita: In front of Lars's tombstone at the cemetery. There will be a brief cutscene where you share a moment of silence with her. The moment will last and the time of day will shift until you press [Interact] again.
Mangus: On top of the large sword in Blade Henge. Note: Use the nearby Landmark Viewer.
Ophelia: In front of the large stone cross near The Hunter in the Grasslands (located to the left of the huge sword on the map). Eddie will ask if she wants to make out. A cutscene will play of the two kissing until you press [Interact] again.
Rima: On top of her tower in the jungle
Thunderhogs: At the Kill Master's Lair. You can drink with his bassists and comment on Lemmy's bass playing ability.

Riding the animals - All animals as big as Eddie or larger can be ridden. Hit a creature with a bolt of lightning to stun them. Then, get near them, and press Y. While riding them, press A to perform an attack specific to that creature. Raptor Elk and Tullusks are a couple examples of creatures with melee attacks, while the Laser Panther shoots lasers from its eyes. The large wooden horse creatures in the final map area (with large swinging axes for tails) can also be ridden.

Kyle Gass reference - The man at the Mortar missions has the same facial features, hair, and voice as Kyle Gass. Kyle Gass is the lead guitarist and vocalist of Tenacious D and the Godfather of Jack Black's son. -From: Tyler Murphy

Psychonauts reference - In the first cutscene, where you are given the options to toggle the blood gore and swearing, just before Eddie says the line that leads to the option of turning on or off swearing, he sees one of the band members dancing on a very shaky ledge. While that band member is dancing on that ledge, he will be chanting a song that was originally done in Psychonauts by some kids in a few parts, but most notably by Raz himself in the level where you have to fight four wrestlers to get a card. At the end of each match after they were defeated, Raz did the same dance and chant song.
The headbanger that wears the goggles at all the Death Rack missions sounds identical to Raz, but slightly deeper.

Rob Halford reference - Rob Halford plays General Lyonwhite even though his last name is Lita and Lars'.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Got a Car and a Date (15 points): Completed "Welcome to the Age of Metal".
Start a Revolution (15 points): Completed "Exploited in the Bowels of Hell".
Thick as a Baby's Arm (15 points): Completed "Lair of the Metal Queen".
Hair Remover (15 points): Completed "Battle for Bladehenge".
French Kiss Instructor (15 points): Completed "Pilgrimage of Screams".
Chicks n' Booze n' Stuff (15 points): Completed "Sanctuary of Sin".
Ran to the Hills (15 points): Completed "It's Raining Death".
Fistfull of Fog (15 points): Completed "Dry Ice, Wet Graves".
No More Tears (15 points): Completed "Sea of Black Tears".
Groupie (25 points): Completed Campaign mode on Easy.
Roadie (25 points): Completed Campaign mode on Normal.
Legend (25 points): Completed Campaign mode on Brutal.
Overkill (20 points): Completed all hunting secondary missions.
Squeal Like a Chicken (20 points): Completed all racing secondary missions.
Protector (20 points): Completed 20 ambush, outpost defense, or mortar secondary missions.
Metal God (50 points): Achieved 100% completion on the stats screen.
Serpent Samaritan (10 points): Freed 40 Bound Serpents.
Serpent Spanker (15 points): Freed 80 Bound Serpents.
Serpent Savior (25 points): Freed all 120 Bound Serpents.
One Hit Wonder (5 points): Purchased 1 upgrade in the Motor Forge.
Loyal Customer (25 points): Purchased all upgrades in the Motor Forge.
Virtuoso (20 points): Learned all guitar solos.
Tourist (20 points): Viewed all vistas.
Now You Must Tell the Tale (20 points): View all Legends.
Voices From Beyond (20 points): Unlocked all songs in the Mouth of Metal.
Flowerslave (20 points): Summoned all Motor Forges.
Practice Bloody Practice (10 points): Won an AI Stage Battle - any difficulty.
Iron Fist (20 points): Won an AI Stage Battle - Brutal difficulty.
Victör (10 points): Won a ranked multiplayer match.
Subjügator (20 points): Won 10 ranked multiplayer matches.
Cönquerer (50 points): Won 50 ranked multiplayer matches.
Master of the Flame (20 points): Double Teamed with every Ironheade unit.
Master of the Tear (20 points): Double Teamed with every Drowning Doom unit.
Master of the Blood (20 points): Double Teamed with every Tainted Coil unit.
Armchair General (20 points): Won a Stage Battle by yourself against the AI without attacking.
Favored (30 points): Acquired 3,000 Fire Tributes.
Sellout (30 points): Spent 250,000 fans - any mode.
Some Demon Flesh on your Bumper (20 points): Slayed 150 enemies with the Deuce - any mode.
I've never touched an axe before (30 points): Personally smote 300 enemies - any mode.
Silence, groundwalker! (10 points): Gained 5 or more seconds of hang time in a single jump - any mode.
Six Degrees of Schafer (15 points): Played with or against another player who has this Achievement.
Ringleader (15 points): Trapped 15 enemies in one ring of fire with the Fire Baron's Double Team.
Painkiller (15 points): Killed 25 enemies with the grinder of one Rock Crusher.
Dollpocalypse (15 points): Hit 6 enemies with the explosion from a Brood's Double Team.
Euthanasia (15 points): Hit 15 enemies with one Agony Ball using the Pain Lifter's Double Team.
Death From Above (15 points): Killed 20 enemies with one Bleeding Death.
Coolest Thing Ever (15 points): Jumped over a Hextadon in the Deuce.
Beast Master (15 points): Rode every animal in the world.
Boar Bather (50 points): Rode a Razorfire Boar into the Sea of Black Tears and live to tell the tale.
Quill Tosser (20 points): Killed a Tollusk using only Ground Urchins.

The following achievements require the "Hammer Of Infinite Fate" bonus downloadable content:

Carnivore (10 points): Run down 20 animals in the Deuce with the Coiling Maw equipped.
If You Want Blood, You Got It (10 points): Kill 20 Tainted Coil units with the Disgorger.
Cry Me a River (10 points): Kill 20 Drowning Doom units with the Eye of Sorrow.
Beard Beard Action Beard! (10 points): Make out with Ophelia while wearing the Mountain Man Threads.
A Face Like a Beast (10 points): Kill 15 enemies with a Metal Beast while wearing the Zaulia Threads.
Hammer of Infinite Fate Tourist (10 points): Play a multiplayer game on all Hammer of Infinite Fate multiplayer maps.
Hammer of Fate Champion (10 points): Win 10 games on Hammer of Infinite Fate maps - ranked or vs. the Brütal AI.
Whispering Rock (10 points): Chisel four Razputin heads onto Mt. Rockmore.
Norwegian Sendoff (10 points): Visit the grave of a fallen hero while wearing the Black Metal Threads.