If you're under thirty, you could be forgiven for never having heard of scene demos. Originally they were simply intro screens with sparse animation that accompanied pirated games when you ran them, to claim credit for whatever cracking team was responsible for defeating the copy protection and distributing the game to the masses. This early, basic type of demo is now called a "cracktro".
Of course human nature being what it is, the various groups, or "crews" began to competed with one another to create flashier, more technically impressive intros. Then, this practice broke away from piracy entirely and demo crews began to code their own standalone pieces simply to show off their programming skills.
The focus narrowed over time to achieving the most remarkable visual effects possible on the platform, while using up the least possible amount of space on disk. As a consequence, the demo scene became a hotbed for innovation in the area of data compression.
Some stars emerged pretty quickly who are still the kings of the demo scene all these years later. Fairlight and Farbrausch for example (best known for The Popular Demo). In Europe, demo parties became an annual tradition. Demo crews would show up for drinking and festivities, but mostly to show off their most recent work on the big screen in hopes of taking home the prize money.
Imagine the feeling of camraderie...and rivalry! The excitement to see what insane new heights computer graphics will be pushed to this year! There's also something of punk culture to it, as demos have been coded even for outrageous, silly platforms like graphing calculators, the old ipod mini, Virtual Boy and basically anything with a screen that can accept outside programs.
The first demo that made a lasting impression on me, and still regarded as one of the most groundbreaking demos of all time, was Second Reality by Future Crew. You can see a video of it here, but it's just not the same as watching it unfold on an authentic 386 or 486 PC.
Besides the expected high color, hand drawn 2D artworks, there was really cutting edge realtime polygonal graphics which look superficially similar to Star Fox but which exceed it in terms of onscreen polygons and framerate. Being able to see your own mundane home computer generating what, at the time, was a feat bordering on wizardry was quite thrilling. Fully 3D explorable game worlds were still some years off, so to get an early taste of what we'd later enjoy in games felt like a glimpse into the future.
This was back when everybody thought virtual reality was just a couple years off, even though decent realtime 3D polygonal graphics on home computers were still out of reach, and the Doom was still about as good as "3D" got on consumer hardware.
However as the years went by, hardware grew faster and cheaper, and the demo scene was always the first to probe its limits. One of the most stunning demos of the 2000s, for me, was Iconoclast by ASD. It should give you some idea of how far ahead of the gaming industry demo coders were with respect to squeezing graphical horsepower out of the hardware available then.
Another demo that really blew me away is Your Fingers So Gently On My Skin, by Plastic. For a demo released in 2004, three years before the Xbox 360 came out, it should be clear that it is a visual marvel. That's to say nothing of the artistic merit!
The modeling, the fish movement and the soft lighting in the underwater scenes still astonish me. It really shows how dreamlike visions can be brought to life through scene demos, and made all the more amazing because they are running in realtime on your computer (or other device), not pre-rendered.
By other device I mean your android phone, though demos exist for every machine with any sort of display. Every game console and handheld ever released has demos for it. But the finest Android demo I've seen yet is d159 by simppafi and gloom. If you have a reasonably modern phone you owe it to yourself to download the apk, sideload it, then check it out.
I first ran this on my Galaxy S5, though I have since upgraded. It will really show off what your phone can do (provided your phone runs Android) and in places, made my eyes bug out as I wondered how they achieved some particular effect. It brings to mind the first scene demo I watched on my old Pocket PC back in the day, boggling at how they achieved realtime stencil shadows on such meager hardware.
Even if your PC isn't up to snuff, you can simply download demos from a couple years back and be fine. Pouet.net has the largest single repository of scene demos on the internet, neatly organized by the device they run on, the year they came out, their popularity and so forth. Armed with this knowledge, go forth and experiment. What devices in your house may already have demos written for them? Let me know in the comments what you find!
Some of my favorites for PC (besides those already listed): 1995, agenda, elevated, fr-043, happiness, nowhere, Rupture, Stargazer, The Product
Some faves for Gameboy Advance: Who's Your Favorite Clown, Berlin, Virus, Newton, Phloam, Five Finger Discount
For Virtual Reality: Surge (Oculus/Vive) Stor Eiglass (Gear VR)