Formal:
* Am delighted to
* Am eager to
* Am pleased to
* Would be honored to
* Would be privileged to
Informal:
* I'd love to
* I'd be thrilled to
* I'm all for it
* I'm down for that
* Sounds good to me
* Count me in
* No problem
More specific:
* I'd be happy to help.
* I'd be happy to take on that responsibility.
* I'd be happy to answer any questions you have.
The best alternative will depend on the specific situation and your relationship with the person you're talking to.