There's also Eaglercraft, which achieves the same result, but it has functioning multiplayer, and runs up to version 1.8.9, though you'll need a beefy computer.
replies(2):
Why is this so exciting to me? I started playing Minecraft back in the Java modding days, which really picked up steam around 1.2.5 and, by my estimations, peaked around 1.7.10. There was a veritable cornucopia of mods widely available, to enhance the game in a plethora of ways. I modded my game so hard I undoubtedly ran more third-party code than first-party/Mojang. Minecraft was a platform for creative expression, freeform and unconstrained. The golden age of Minecraft modding!
But it wasn't all unicorns and rainbows. There was a dark side of modding: the ease of use, or lack thereof in getting mods setup. Modpacks helped, but you still had to run untrusted code, and you still had to have a compatible PC. This is what (nearly) killed Minecraft for me. I wanted to play with my friend, but his PC had Windows upgraded and lost Java or OpenGL compatibility, and he eventually dropped his desktop and switched to tablets, phones, and consoles. My highly curated developed modded technical world became inaccessible.
Contrast this unfortunate situation with the browser: with "load and go" functionality as Douglas Crockford calls it, you can just visit a website and play, provided sufficient specs. What if we could play modded Minecraft (preferably 1.7.10), with all the mods to our hearts content, right in the browser, accessible to anyone?! And anyone could develop and distribute their own mods without concern, being safely executed within the browser sandbox.
I don't know how close Eaglercraft is to this utopia, but I find the possibility very enticing.