Formal:
* Yes
* Certainly
* Absolutely
* Indeed
* Affirmative
* Of course
Informal:
* Yep
* Yup
* Sure
* Okay
* Right
* You bet
* No problem
* I agree
* You got it
* Absolutely!
* Definitely!
Slang:
* Yeah
* Yah
* Uh-huh
* Mmm-hmm
The best alternative for you will depend on the situation and who you are talking to.