General Opposites:
* Calm
* Content
* Happy
* Pleased
* Satisfied
* Unbothered
* Apathetic
* Unconcerned
More Specific Opposites:
* Complacent (satisfied with oneself or one's situation)
* Submissive (accepting or yielding to the will of another)
* Resigned (accepting something unpleasant that one cannot do anything about)
* Tolerant (accepting or allowing the existence of opinions or behavior that one dislikes)
* Unoffended (not feeling offended)
* Forgiving (willing to pardon or overlook a fault)
The best antonym will depend on the specific context in which you're using the word "indignant."