[deathcap]

> Or let me put it differently - what's the most popular open source client?

Another open source Minecraft-compatible client in the works is https://github.com/thinkofdeath/steven. Written in Go, already decently functional, though still far from finished it is under active development and progressing nicely. Not that popular yet but seems to be promising to me. The author is a major contributor to one of the most popular Minecraft server software packages.