Mmm, hi there ^_^
Greyhawk has always been the default setting for D&D, from its original days all the way through the latest (4th) edition. More appropriately, the default world is called Oerth, but the campaign itself it called Greyhawk.
Forgotten Realms is very popular (and my personal favorite). There are books from 2nd and 3.X editions. In 4th, they consolidated all the separate campaign settings into (for now) just a campaign setting and player's guide books. It is a very deity-centric world where the gods are extremely active and there are a large number of them. The world is known as Faerun.
Eberron has been around since 3.5, and in 4th is currently just two books, like Forgotten Realms. It is more of a mixture of pulp and high fantasy. It is a very intriguing world. Eberron is also the name of the world.
And the last one of the more popular ones is Dragonlance, which I seen you know about already. :)
There are also several other D&D worlds. Ravenloft, Kingdoms of Kalamar, Al-Qadim, Birthright, Dark Sun, Hollow World, Lankhmar, Mystara, Planescape and Spelljammer. Ravenloft existed from 1st to 3.x editions. Kingdoms of Kalamar is around now, starting back in 3rd. The rest were all 2nd edition games that have seen user support throughout the years of no huge publication. Ravenloft and Dark Sun will be making appearances in 4th edition later this year. :)
I hope this breakdown helps you a bit ^_^ If you have any other questions feel free to PM me :) As far as here on E, I honestly do not know what is popular and what isn't.