How to discover new popular keywords ?

not sure but i don't believe in keyword ranking. keywords drop up and down.. some webmaster are successful with the keyword targeting.. i personally haven't had any luck in this case

i am not aware of any such tool

KeywordDiscovery compiles keyword search statistics from over 180 search engines world wide, to create the most powerful Keyword Research tool.KeywordDiscovery can tell you the search phrases people use to find products and services, as well as the search terms that drive traffic to your competitors.

which means at least 100 people will use the same keyword for the same stuff.. which means.. back to the same question... effective keyword to get first in the place.... ????

there are many sites which gives top searched keywords their rankings etc etc like there is overture from yahoo. you can check these sites to get the top searched terms.