Here are two fine examples of worldbuilding on the internet.
The catalogue of UK Entrances to Hell is exactly what it says. It is an amateurish site, banner ads and garish colors and all, looking like a something about good fishing spots except it's about entrances to hell. Browsing through the deftly-named locations, a bizarre world is created in the reader's mind through the innocuous use of telling details:
"Benidormo is made of glass and is where the devil famously had both of his wrists broken by Sinbad 12 centuries ago. Benidormo has a heartbeat and a robot arm. Radiation trace: minimal"
"Reserved specifically for use in wartime, 54 can be submerged in three seconds and has barriers of thick steel in the approach corridors. Since mediaeval times the monks of Bersgedd Minor have focussed their assaults on the devil mainly here at 54, except during heavy rain, when they would prefer to attack the much less important Little Elwick. 54's tunnels to the Core have never been mapped, even by The Rice-Makers Dictatum. Radiation trace: negligible"
Because the information is so minimal and inconspicuous, and because the content of the site cannot truly be disproved (unless you live in the UK I suppose) the mind fills in the blanks and the illusion of browsing a serious site about a subject you apparently missed in the news and in history class is complete.
The other example is the House of Clocks. Have a look through the website of this quaint museum in Chicago.
Feels real, doesn't it? They sure have a lot of clocks. Now look where they are.
"The House of Clocks may be found at #12 Meat Street, in the Shambles district of Chicago."
An odd but plausible address, wouldn't you say? But perhaps there is more than meets the eye.
Now read the guestbook.
Cool, huh? Again, a plausible source of information contains hints to the otherwordly and mysterious. There is just enough to make you trust, not enough to make you disbelieve, and plenty to make you wonder.
Game design, assuming your game has some kind of narrative context, involves creating a fictional world in the head of the player. These sites show some effective techniques for doing that.