No, this is not a old addon or a RTB addon (that I know of). This is a addon request/idea.
What the physgun does:
Add a bit of physics to the game. You can move blocks around by holding left click on it. If you hold E and move your mouse around, you will be able to rotate it. This can be used for random things I really don't know what posers do with.
You can also freeze them, which forces them to stay where they are, floating or not floating, by right clicking while you are grabbing it with left click. If you want to unfreeze it, simply grab it again.
I don't know if Blockland can actually do this sort of thing or get mildly close to it, but still, let me know what you think of making this as a Blockland addon.