1. Exactly.
2. Absolutely.
3. That's so true.
4. That's for sure.
5. I agree 100%
6. I couldn't agree with you more.
7. That's exactly what I think. / That's exactly how I feel.
8. (informal) Tell me about it! / You're telling me!
9. (informal) I'll say!
10. I suppose so. (use this phrase for weak agreement – you agree, but reluctantly)
Note: Avoid the common error of saying "I'm agree" – the correct phrase is "I agree" or "I'm in agreement" – the second one is more formal.

Fact
It takes seven years for a lobster to grow 1 pound      .. More >>