There's a Rust client in the making called Hematite[1], but it currently will only allow you to move around static worlds, with no other interaction. The focus so far has been on making the rendering as close to pixel-per-pixel identical as possible: [2]

[1]: http://hematite.piston.rs/

[2]: http://imgur.com/a/bHcLD#0

I've made this some years ago: https://github.com/banthar/mine3d

It's just the basics - only renders terrain and some static blocks. But it's also based on 1.7.3 protocol.

I've just checked and it kinda works with TrueCraft server: http://i.imgur.com/Brf2NXg.png

What license is that under?

> 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.

There are no popular open source clients. Hopefully TrueCraft will become one.

Huh - so, I'd wager that Minetest is the most popular open source thing that's KIND OF like a Minecraft client, but your point is that it's not ACTUALLY a Minecraft client, right?

Minetest is a Minecraft clone.

TrueCraft is a Minecraft implementation.