There are a lot of pre-packaged worlds available, but some of the richest are home-brewed, and sometimes they can lead to the creation of entire new games. Discussions about building worlds, from civilizations to laws of physics (and possibly metaphysics), recreation of existing fictional worlds, and modification/combinations of existing worlds. This can include, but is not limited to, creating solar systems, star clusters, galaxies, and parallel universes.
World Building, my favorite subject. The first thing I have to say about this is start small and work your way out. My characters started in a town, a small town. I decided this town was having a festival, during the festival people began disappearing. I decided they were disappearing into a demi-plane created by a dying wizard. This wizard, and his cohorts, were responsible for the shattering of the entire planet. He was seeking resurrection through his descendants and the only way to do that was to put his magical 'feelers' out through knickknacks sold during the seasonal festivals throughout the continent. The player characters systematically found and defeated the ancient wizard and so began the story that actually ended up being the basis for the novel I am attempting to get published.
All of that was session 1, so you can imagine where the campaign grew from there.
One thing I have to say about world building is to diversify the people of your lands but don't go overboard. Not everyone is stuck in the medieval European time line. Take your cues from other civilizations right here on earth. The Japanese, the Maya, India, Native Americans, etc. I own so many history an 'mysteries of the world/past' books it would spin some peoples' heads. I consult those books for help with building a people and even plot hooks from time to time. Once you have a good structure try out the http://www.cartographersguild.com/ for help with maps and all that.
So to expand on the world as an example (this was back in 1998) I decided that the world in question was going to consist of 8 known and one unknown continent floating out in my version of the astral plane, the astral fade. Approperately put, the metaphysics involve raw elements holding the continents together and slowly mending the rupture of the planet. Once you leave the 'formally inhabited' lands of the continents you can actually see the astral fade from the landmass in question. If you were standing on the underside of the continent (my version of the interconnected astral underdark), you could see the vast expanse of the astral fade above your head. Very disconcerting.
A spin like that is what takes your campaign up a level and gives your characters something to think about. Hopefully you'll all get to read about it once my editor is done. lol
My most intricate world building to date was/is the one fantasy world we game in. It's system agnostic, really, where we started it in AD&D 1e, converted to 2e, left for Palladium Fantasy because we got sick of D&D not making skills useful, converted back to 3e because skills suddenly were relevant, played through to 3.5, then got turned off completely and now run it in Pathfinder. But the world itself is consistent. I've also used worlds for one-shot campaigns using various game systems.