Singleplayer Commands

A cave with netherrack replacing stone and lapis lazuli replacing water

Singleplayer Commands is another great mod that allows you to use commands, like in multiplayer, while playing singleplayer mode. It also includes the world-editing tool WorldEdit that allows you to make large-scale changes to your world. For example you could select a large area, and then replace all stone with netherrack for your amusement. It is also very handy for large builds as you can build a huge wall in just seconds.

It is available for download here:
Single Player Commands