MC Edit otherwise known as World Edit is a software that enables you to edit your world without actually mining block by block. So it is usually used when you need to flatten the landscape or build a platform.
Sadly the 'Code Warrior' (the creator of MC Edit) stopped developing the program. it doesn't really work now. 'Code Warrior' has uploaded a post saying that he will stop developing MC Edit and the responsibility of developing and updating MC Edit is now 'Tk Tech's' Job. Here is the link to that post
http://www.minecraftforum.net/topic/13807-mcedit-minecraft-world-editor-now-open-source/
He also states that only unstable builds are available now and that you should use at your own risk. I used to really enjoy using MC Edit :( the day I read the post I felt depressed..........
However there is an alternative, although it is not as good as MC Edit a program called WorldPainter still does same job. So here is the download link to WorldPainter, and MC Edit.
WorldPainter:
http://www.worldpainter.net/
MC Edit (unstable version. you cannot get a stable version for now I will inform you when the new version comes):
https://github.com/mcedit/mcedit/downloads