HTML parody of an OS which was popular when we were young and Internet was new
It's the first time I got to know about something like this, but looks like this stuff existed for some time: a website mimicking Windows OS.
The one I saw first and got impressed by is windows93.net. It is actually a collection of javascript games, decorated as applications and spiced with some humour.
Also it was fun to see "name fails" in "P()rn" section in Cat Explorer (Internet browser).
Source: HN.