I like using Notepad2-mod (see useful software, notepad2-mod webpage) to edit OOMMF mif files, but I do generally find it a lot easier to edit if the syntax is highlighted. Since mif files are essential tcl we can just tell notepad2 to treat mif files as tcl files for the purposes of highlighting:
View -> Customize Schemes
Scroll down to Tcl Script.
In the “Associated filename extensions:” box add (without quotation marks) “; mif” to the existing entries.
Now when you open mif files in notepad2-mod you should find the syntax is highlighted by default.