The most common, of course, is the "mm" that we use to say,a "yes, I am paying attention to what you say". This could be drawn out into a "hmmm" to signify, "Yes, I agree with you, but what can be done about it?" Or it could be a question..."mmm...?" ("Oh, I'm sorry, what did you say?")
But the most interesting sound, to me is "pstchp" (a poor phonetic rendering of that sound made with pursed lips).This often signifies two opposing things...agreement and disagreement! We often use this when we see something not being done right, prior to doing it properly ourselves and showing the other person; or, in the face of some unpalatable fact (eg. someone says, "She was a nice lady, I wish she'd lived longer") the sound is made, expressing both regret and agreement.
We call it "tchooL kottarathu" in Tamizh...what is it called in other languages? I don't find this sound being made in Western cultures...is it a purely Eastern thing?