IE is by far the most popular but is widely regarded as a gaping security hole.
I think that Opera has adverts unless you pay for it. I've never used Netscape Navigator but I have used Mozilla Firefox and think it's smashing.
You cant forget firefox.. i have used all the browsers and firefox is generally better than all of them.
i dont know if IE is the most popular as such, i think that iff ppl were educated more int the ways of firefox or mozilla and truely given the chance to use them, then IE would die, but as it is thats what you get when you buy XP and as long as ppl are lazy and unwilling to try new things IE will prevail. Pah..