Top 10 Free Mastering Plugins 

1. Youlean Loudness Meter

2. Voxengo Marvel GEQ

3. MAnalyzer by Melda Production

4. Limiter No6 by VladgSound

5. Voxengo SPAN

6. Ferric TDS

7. Ozone Imager 2

8. TDR Nova

9. Flux Stereo Tool V3

10. MV Meter 2

Down below is a short description of each of the mentioned plugins.

1. Youlean Loudness Meter

The free version of a plugin of the same name, the Youlean Loudness Meter gives you a highly accurate measurement of the perceived loudness of your audio. Follow the history of the loudness in your edits and pinpoint any and all problems in your song; resize and scale the plugin to your specifications, and take control of the loudness in your tracks with ease.
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LALAL.AI – Special Feature

LALAL.AI, the next-generation vocal remover and music source separation service that revolutionizes stem extraction. LALAL.AI offers a seamless and precise method to extract vocals, drums, bass, piano, electric guitar, acoustic guitar, and synthesizer tracks from audio files without compromising quality. Users can start with a free trial to experience the power of LALAL.AI before opting for an upgrade, allowing them to process more files and receive faster results. The service provides different packages such as Lite, Pro, and Plus, each with varying minutes and upload size limits. Additionally, LALAL.AI has expanded its services to include Voice Cleaner, a feature that effectively eliminates background music and noise from voice recordings. For user convenience, LALAL.AI offers both a web-based service accessible from any device with an internet browser and a dedicated desktop application available for Windows, macOS, and Linux. LALAL.AI is the go-to solution for music producers and creators seeking seamless stem extraction and pristine voice-cleaning capabilities.
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2. Voxengo Marvel GEQ

Marvel GEQ from Voxengo is a linear phase 16-band graphic equalizer with multi-channel operation support. Simple, quick, and easy EQ adjustments, not just to entire songs, but for full tracks as well. Comes with a variety of color variants and customization options to allow you to work your way
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3. MAnalyzer by Melda Production

MAnalyzer, from Melda Production, gives you a spectral analyzer and sonogram that measures average, infinite average, temporary maximum, and an infinite maximum of remarkable quality for a free product. Use super-resolution to provide high resolution in all kinds of frequencies or de-harmonize to find relative frequencies.
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4. Limiter No6 by VladgSound

The aptly and simply named Limiter No6 from VladgSound allows for flexible tuning, high quality signal processing, support of mono/stereo and multiband modes, up to 4 times oversampling, five modules, and true inter-sample peak limiting. Simple and effective.
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5. Voxengo SPAN

SPAN is the real-time “fast Fourier transform” audio spectrum analyzer from Voxengo. Flexible thanks to a mode system which allows you to customize your spectrum analyzer preferences, SPAN puts easy and effective spectrum analysis at your disposal free of charge.
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6. Ferric TDS

Looking to access tape-style dynamic effects for your project? Look no further than the Ferric TDS, which gives crunch and presence to your synths or add subtle compression to everything from guitar to drums. It’s a neat, easy to use way to give some vintage flavor to your tracks without breaking the bank on more expensive software.
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7. Ozone Imager 2

Made by world-renowned plugin developers Izotope, Ozone Imager 2 is extraordinarily complete for free software. Easily adjust the width of your track, “stereoize” it to automatically convert it from mono to stereo without a hitch, and visualize your width and alignment through three different vectorscope meters.
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8. TDR Nova

Nova is a free to download parallel dynamic equalizer from TDR, or Tokyo Dawn Records. This software pretty much does everything you could want from an EQ, capable of parametric equalization, dynamic equalization, frequency selective compression, multi-brand compression and wideband compression. It also happens to be as polished, smooth, and stylish as plugins that go for hundreds more.
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9. Flux Stereo Tool V3

Described by makers Flux as the complete stereo imaging and analysis tool, Stereo Tool V3 could very well be just that. It offers ultra-precise control of input gain and individual pan for left and right channels, accurate visual feedback, a vector scope, PPM meters for inputs/outputs, phase correlation meter and individual left and right polarity reversal- all absolutely free.
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10. MV Meter 2

A multivariable meter that does RMS, EBUR128, VU and PPM measurement, MV Meter 2 expands on its already great predecessor by adding single/dual meter display, adjustable reference level for all meter modes, adjustable meter delay and customizable presets.
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Image credit: Melda Production