What is Glazz?
Glazz is a mod API for the Minecraft client. It's similar to Bukkit in many ways, so if you're familiar with Bukkit, just think of it as Bukkit for the clientside. I'm creating this out of a lack of a good modding API (ModLoader allows loading mods, sure, but it doesn't implement an entire API, and mods can cause conflicts very easily if they both modify the same file).
So what does Glazz let me do?
Glazz lets you create almost any plugin you can imagine - a world editing plugin, a HUD plugin, singleplayer mods, and almost everything else.
What parts are there to the Glazz project?
The Glazz project contains of two seperate parts.
Glazz is the API itself, and is what you will reference to create a plugin. CraftGlazz is the modded Minecraft client, which is able to run Glazz plugins.
Why not just use Spoutcraft?
Spoutcraft, from my understanding, requires a connection to a multiplayer server to use mods. CraftGlazz does not.
I WANT TO DOWNLOAD IT!
Okay, okay! Fine! CraftGlazz is still a development build, so it may be unstable (and it's certainly not even close to feature-complete at this point), but you can download it from Jenkins!
How do I install it?
First, ensure that you have a clean minecraft.jar file (by using the "Force Update" option in the launcher).
Open your minecraft.jar and the craftglazz-dev-SNAPSHOT.jar and place them side-by-side.
Delete META-INF from minecraft.jar.
Drag the contents of craftglazz-dev-SNAPSHOT.jar into minecraft.jar (including META-INF).
Congratulations, CraftGlazz is now installed!
What does it do?
CraftGlazz just allows you to run plugins (which can be put in your .minecraft/plugins directory). It also includes a built-in console - press ~ (tilde) ingame to use it.
I want to code plugins for Glazz. How do I do so?
First, go learn about coding Bukkit plugins if you don't already know how, as coding Glazz plugins is extremely similar.
Next, to code a Glazz plugin, simply reference the Glazz (not CraftGlazz!) JAR file in your project. Have your plugin's main class extend "Plugin" instead of "JavaPlugin". Be sure to have a plugin.yml, and you're set!
With the way the buildbot currently works, it redistributes an internally renamed minecraft.jar file. This would obviously allow someone to run the game without purchasing it (not that they'd be able to join any online servers, however).
*ignoring the starting spam fest of the child "gamers"*
I do hope this works out like you specified in the OP this would be rather nice.. Although when the official API comes out it might be rendered useless.. (I hope so) And I do not mean that in a negative way towards you, but Minecraft needs an official working API.