MPfastfill

The mod adds one tool that can't be crafted; it can be equipped in any creative tab. It's designed for quickly filling an area.

Here's a quick guide:
Press Shift + RMB on a block with the tool in hand to select the block to replace (you don't need to do this every time you replace it).
Press RMB on the first position.
Press RMB on the second position, and the entire cube will be filled with blocks.

Also, when filling, you'll receive a chat message indicating the command entered.
Disadvantages of the mod: The tool's actions cannot be undone.

Very convenient for building maps and filling areas; no commands required, it's simple.

The current block is displayed in the bottom left corner of the screen when you have a tool in your hand.

The mod requires no dependencies; enjoy!