Minecraft Infdev Ultimate

Early Classic+:

Early Classic+ is a mod for 0.0.13a_03 that adds multiplayer support, new blocks, and other cool things.

Singleplayer: Play, or Download the client

Minecraft Infdev Ultimate:

Download the mod here, or Download the server

It's got multiplayer, some other improvements, and it's even more stable than Alpha servers!

News:

Infdev Ultimate is currently nowhere near finished, but you're free to try it.
Keep in mind that it's under heavy development and can be a bit unstable during updates.
You can read more on the discord server, or here.