Marvel: Ultimate Alliance Codes -

Play as Iron Man:
Fully complete (through level 10) two of the S.H.I.E.L.D. simulator missions.

Play as Silver Surfer:
Fully complete all four S.H.I.E.L.D simulator missions.

Striker combos:
Summoning certain Striker characters with specific characters will yield different results:

Summoning Dr. Strange with Blade makes him generate a demon animal which does much more damage.

Summoning Namor with Miss Marvel makes him turn into a shark, doing much more damage.

Summoning Iron Man with Captain America makes him put a shield around you.

Summoning Phoenix with Iceman makes her drain your health.

Ghost Rider and Silver Surfer do not appear to have any combo partners.