Bucket-fill tool (world edit)

[IntegratedQuantum]

Pretty basic tool, copied from GIMP
Right click to select a block to fill, left click to replace all connected blocks with selected block

[Argmaster]

I would suggest a different approach - allow flexible (non-rectange) selections instead and offer fill-select instead. So we can achieve more results with already existing commands.
So, magic selection tool left click + shortcut for picking up a block pattern with a brush tool.
Could be better suited as long term solution since we are quite far from implementing it, but this is the route modern world editing software seems to be taking (eg. Axiom).