Above, one of my favourites, Kindercrasher by Rgba in the 4k (yes, four kilobytes) category.
Insert floppy. Mind you, you are going to need Windows for this. And I would not be too confident running this stuff in emulation as it is all real time graphics and GPU squeezing to the max. If you dig around, there are some demos for “older” systems as well: VIC20 and C64. Hearts to the demoscene.