And the latest entry in the annals of hobby operating systems:
MenuetOS, an OS whose main feature is that it's written entirely in assembly language. The product of an intrepid cabal of European hackers/masochists, it has a GUI that looks sort of like Windows XP would look if it didn't have proper widgets and was limited to one pixel font, and all the apps anyone in the target audience would need (i.e., it has an IRC client, a NNTP newsreader, MP3 playing/streaming capabilities, a chess client and a Doom-style 3D engine demo), and fits onto a single floppy.