This is, without question, the best essay -- indeed the best anything -- I have ever read on the internet.
It's basically a trip through the development of the computer industry along with arguments around what will, or will not, still be around in fiftenn years. The whole thing is anchored in a discussion about User Interfaces specifically, and operating systems in general. It's not written from a tech perspective, per se, so even lay people can understand it.
It's very long; it took me around three hours to read it all. So long that I'm sure few people ever read the whole thing. When discussing online documents, very rarely does one think of anything larger than 10 pages. This one works out to around 75 pages.
It's also a little anachronistic. It was written in January of '99. In those days, Windows 98SE was new and 1GHz was unheard in microprocessors. Yet it still reads very clearly and is very relevant in this day and age.