Nandri

Tamil equivalence for "Thanks/Thank you" in a very respectable and hearty manner of conveying thanks.
Usage is exactly as you would use Thanks or Thank you.
Also used as: Mikka Nandri, which is a superlative of Nandri.

X: What you did was remarkable, keep it up.
You: Nandri !! (or Mikka Nandri)