How to modify my voice using a program?

I'd like to modify my voice using a program (it doesn't matter if it's free or expensive). For example listening to the new Black Eyed Peas hit "Don't stop the party" in 0:26 it says Ey my people, this is mega.. or something but it's Will.i.am voice and the music video you watch him speaking .. how do you get that effects using a program.. And what program should I use? Thanks!