These scripts are written in Python, and add various features to a Minecraft savefile. They behave like an automatic editor, altering the contents of an existing world. They do not change the way that Minecraft behaves in any way, and are not considered a mod.
Download both the script and the interface module to your "saves" folder. Edit the script to set the options. Run it to edit the save file. See the appropriate forum thread for more details.
In order of most recently updated:
WizardMountain makes floating mountains from existing terrain. 11/22/2011
WizardMountain instructions, help, discussion thread
LineRail makes powered rail tracks 11/18/2011
LineRail instructions, help, discussion thread
StarStone makes craters and boulders 11/17/2011
StarStone instructions, help, discussion thread
Forester makes epic trees. 11/16/2011
Forester instructions, help, discussion thread
mcInterface the interface module for Beta files, you need this for all beta scripts 11/17/2011
These are extra wrapper modules to make the above scripts work as filters in MCEdit by Codewarrior. You must copy the script ("Forester.py" for example) as well as the wrapper (MCEditForester.py for example) and the interface file ("mcInterface.py") into the "MCEdit\MCEditData\filters" folder. The filter should show up automatically inside MCEdit. Be sure that all three files are present for each script.
The selection box is absolutely respected, so be sure to select a large enough area for the script to do its work. These scripts take about five times longer to run in MCEdit than they do as standalones. If you want it to go faster, use the script as a standalone. If you prefer a GUI, use them inside MCEdit.
LineRail filter wrapper. It adds GUI support to MCEdit for the WizardMountain script. 11/18/2011
WizardMountain filter wrapper. It adds GUI support to MCEdit for the WizardMountain script. 11/17/2011
StarStone filter wrapper. It adds GUI support to MCEdit for the StarStone script. Big craters may take a very long time to process. 5/20/2011
Forester filter wrapper. It adds GUI support to MCEdit for the Forester script. 5/19/2011
Altitude makes levels taller or shorter 5/13/2010
Groundwork makes city infrastructure 4/12/2010
coral, a test script that makes coral 4/12/2010
hutman, a test script that makes huts 4/10/2010
MountainFortress makes castles 3/19/2010
MCUnity joins two levels together 3/8/2010