Surge XT is a software synth plugin. Version 1.2 is now released. This version improves upon its tuning functionality, accessibility and other things. An excerpt from the changelog reads:

Major Feature: Tuning Upgrades

• Surge can act as an OddSound MTS provider (‘master’) allowing the Surge tuning editor to provide tuning to an entire session.
• Remediate yet more edge cases in our internal tuning, including keyboard mapping larger than a scale.

The short explanation is, if you are using synths that support tuning via MTS-ESP, Surge XT can now act as the MTS-ESP master, which means that you specify your tuning within Surge XT and then the other synths will follow the same tuning. This is intended to be more convenient than loading the same tuning data into multiple instances of various synths.

Surge XT is free and available on Linux, Windows and macOS.