Formal:
* C'est bien. (This is good.)
* C'est correct. (This is correct.)
* Pas de problème. (No problem.)
* Tout va bien. (Everything is going well.)
Informal:
* C'est bon. (It's good.)
* Pas grave. (No big deal.)
* Ça va. (It's okay.)
* D'accord. (Okay.)
* Tranquille. (Chill.)
To show you understand or agree:
* Je comprends. (I understand.)
* Je suis d'accord. (I agree.)
To reassure someone:
* Ne t'inquiète pas. (Don't worry.)
* Tout va bien. (Everything is going well.)
The best way to choose the right phrase is to consider the situation and your relationship with the person you are speaking to.