Designspace files were introduced by MutatorMath, and are now used by varLib too.
Superpolator is a stand-alone application to design interpolation spaces. It provides an interactive UI with a designspace map, interpolation preview, location editor, rules editor etc.
Superpolator's native data format is .sp3, but it can also read and write .designspace files.
Define your design space using Superpolator's interface.
Choose File > Export DesignSpace… to export the data to a .designspace file.
Superpolator requires a paid license for continuous use. A fully functional demo version is also available, and expires after 30 days.
DesignSpaceEditor is a RoboFont extension which provides a simple interface to create and edit designspace data – without any interactive preview, map etc.
Design your designspace using DesignSpaceEditor's interface.
Click on the Save button to save the data to a .designspace file.
DesignSpaceEditor is open-source and can be installed with Mechanic 2.
Designspace files can also be created with code: fontToolsAn open-source library for manipulating font files with Python. supports reading & writing .designspace files using the designSpaceLib.
DesignspaceLib started as DesignSpaceDocument, a separate library, and was recently incorporated into fontTools.
The script below will create a .designspace file for MutatorSans. To try it out, save it in the MutatorSans folder next to the UFOs.