Microsoft VOLT

VOLT supports a wide range of substitution and positioning types. It also contains a proofing tool for reviewing the application of layout table lookups.

It allows import and export of glyph names, lookups, glyph groups and a full project.
The tool has been used by community members to add OpenType layout tables to various fonts supporting scripts, including Arabic (Naskh and Nastaliq writing styles), Bengali, Cyrillic, Devanagari, Greek, Gujarati, Gurmukhi, Kannada, Latin, Sinhala, Syriac, Telugu, and Thaana.