Here's a list of 10 free mastering plugins for pro audio, in no particular order:

1. Limiter №6 by vladg/sound - a versatile limiter that can be used for mastering, mixing or sound design.

Download: https://www.tokyodawn.net/vladg-limiter-n6

2. LoudMax by Thomas Mundt - a brickwall loudness maximizer that can be used for mastering or limiting.

Download: https://loudmax.blogspot.com/

3. DC1A 3 by Klanghelm - a compressor that is great for mastering and is known for its easy-to-use interface.

Download: https://klanghelm.com/contents/products/DC1A.php

4. TDR Nova by Tokyo Dawn Records - a dynamic equalizer that is great for mastering and offers a lot of control over the sound.

Download: https://www.tokyodawn.net/tdr-nova/

Ozone Imager by iZotope - a stereo imaging plugin that can help widen the stereo image of a mix during mastering.

Download: https://www.izotope.com/en/products/ozone-imager.html

MSED by Voxengo - a mid-side processing plugin that can help with stereo imaging during mastering.

Download: https://www.voxengo.com/product/msed/

IVGI by Klanghelm - a saturation plugin that can add warmth and character to a mix during mastering.

Download: https://klanghelm.com/contents/products/IVGI.php

Youlean Loudness Meter by Youlean - a loudness metering plugin that can help ensure a mix meets the proper loudness standards.

Download: https://youlean.co/youlean-loudness-meter

SlickEQ by Tokyo Dawn Records - an equalizer plugin that is great for mastering and offers a lot of control over the sound.

Download: https://www.tokyodawn.net/tdr-vos-slickeq/

SPAN by Voxengo - a spectrum analyzer that can help with identifying problem areas in a mix during mastering.

Download: https://www.voxengo.com/product/span

There are several benefits to using free mastering plugins for audio production:

1. Cost-effective: The obvious benefit of using free mastering plugins is that they are cost-effective. You can get high-quality tools without having to spend any money. This is particularly beneficial for new or independent producers who may not have the budget to invest in expensive plugins.
2. Wide variety of options: There are many free mastering plugins available online, which gives producers access to a wide variety of options. This allows producers to experiment with different plugins and find the ones that work best for their particular sound.
3. Accessibility: Free plugins are easily accessible to anyone with an internet connection. This means that producers from all over the world can access the same tools, regardless of their location or financial situation.
4. Learning opportunities: Using free plugins can also provide a great learning opportunity for producers who are just starting out. They can experiment with different plugins and learn about the different parameters and settings that can be adjusted to achieve different results.
5. Integration: Free plugins can often be integrated with other audio production software, making them a versatile and useful addition to any producer's toolkit.

Overall, the benefits of using free mastering plugins are numerous, and they can provide a cost-effective and accessible way for producers to achieve professional-level audio quality.