X-Men Origins: Wolverine

Successfully complete the game under the Hard difficulty setting.

Collecting three action figures in a level play to start a challenge against a Wolverine in an alternate costume. Defeat him to unlock the corresponding costume for use.

Blue/yellow Classic Wolverine: Defeat Classic Wolverine in his challenge.
Brown/yellow Legendary Wolverine: Defeat Legendary Wolverine in his challenge.
X-Force Wolverine: Defeat X-Force Wolverine in his challenge.

When you are fighting Victor Creed (Sabertooth, your brother), pick him up, and throw him at the jukebox that is at the top right corner of the room (It will glow green during Feral Sense) Some music will play for a short while. And although when Victor smashes against it, and it appears to fully break, it seems that it auto repairs itself, and you can do it again, and again.

Collect three hidden Classic action-figures to start a Classic X-Men fight. Defeat them to unlock that costume.

Unlock at least one action-figure fight by collecting three Classic, Legendary, or X-Force action-figures.

In the first level, just before you find the helicopter, you will have to push a wall down on three enemies. Two of them will get crushed. The last will start saying something like "me alive, me alive" before wolverine jumps down on him (notice the enemies big eyes) This is a reference to the Crash Bandicoot series, when either Crash, Neo, or a random enemy will say that line, before something comes down on them. They also have those eyes in the Crash games.

After you crash the jeep on the first level and continue, you reach an area with a barricaded gate. Climb over the rocks and go in. Defeat the enemies. Look for some boards over a small doorway off on one of the areas. Slice them up and enter to find the hatch on the ground near the back corner.

When falling during the Boss battle with the Sentinel, destroy the second turbine on the Sentinel's back. Fly through the large O-ring that falls off at the correct moment.

Go to the Alkali Lake section of the Frozen Tundra (before the Agent Zero fight). Immediately after the long road section with the shield troops is a fallen tree. Enter the forest that leads to a cave. Inside will be the sword and skeleton. Collect up the mutagen.