MCEdit is a
Open MCEdit, then choose a saved map from Minecraft. You can then fly around it using the mouse and keyboard, and use the interface along the bottom to edit various aspects of the map. The Select tool allows you to select areas of blocks, then delete, copy, cut and more. The Brush allows you to 'paint' any typ of block into your map, and you can specify the proportions of the brush, meaning you can add huge lumps of material if you want. There's a clone tool, fill and replace, and a filter to use with the select option. It's also possible to move the position of your player, and also set a new spawn point.
MCEdit is pretty easy to use. Editing 3D spaces is always tricky, but MCEdit does a good job of making it as accessible as possible. Everything in the application is well explained if you have a reasonable understanding of Minecraft itself. While purists may think using editors is cheating, MCEdit does allow you to do some cool stuff that otherwise would have been really difficult. One good example is making a huge TNT ball to blow enormous holes in the landscape - it's ugly, but a pretty quick way to mine for minerals!
MCEdit is, in our opinion, the most user friendly Minecraft editor around. It's a great way to add a new layer of creativity to the game!