Sectorforth is a 16-bit x86 Forth that fits in a 512-byte boot sector (2020)

Eight primitives and a colon compiler in 512 bytes is a nice demonstration of how much you can bootstrap from almost nothing. The examples building up from there are the best part.

2020

Not that impressive considering this uses BIOS routines heavily and whole operating systems including basic interpreter fit in a couple of KiB