Freeware - MOD Audio Desktop VST Plugin

https://mod.audio/desktop/#download_app

Fully stocked open-source Guitar Amp Sim and Effects Suite!!

Freeware - Tonocracy Guitar Amp NAM Sim and Effects VST Plugin

https://tonocracy.com/

The ultimate and NOW FREE guitar tone and effects software from Atomic Amplifiers!

Loads NAM Profiles!

Freeware - Mercuriall Ampbox VST Plugin

https://mercuriall.com/cms/details_ampbox

Mercuriall Ampbox is a next-gen VST/AU/AAX + standalone amp modeling plugin for PC/Mac. It's an ever-evolving platform for all* our products, current and future. You will get Ampbox for free when purchasing one of the supported plugins*. The platform has everything you need to create perfect tones with just one plugin. It's simple, elegant and versatile. Powered by Mercuriall Neural Hybrid Engine v2.0 technology. Mercuriall Ampbox free modules included.

Freeware - HeadRush Revalver Free VST Plugin

https://revalver.headrushfx.com/free/

Whether you want to use the ReValver stand-alone version or the VST/AAX/AU plugin in your favorite DAW, it'll cost you nothing. Free version comes with several modules and pedals.

Freeware - Audio Assault Amp Locker VST Plugin

https://audioassault.mx/collections/plugins/products/amp-locker

Amp Locker comes with free gear to get you started The free Prestige Amp, a british hotrodded amp, 7 Pedals, 7 FX and 11 fully adjustable 3D Cabinets

Freeware - Fractal Audio Cab Lab 4 IR Mixer VST Plugin

https://www.fractalaudio.com/cab-lab-4/

For decades, guitarists and audio pros have carefully selected, positioned, and mixed microphones on guitar speakers in the pursuit of great tone. Now you can do the same, and it is easier than ever before. With Cab-Lab 4, you get a 4-channel IR mixer, plus tools to EQ, align, and more, so you can select, position, and mix mics while listening in real-time. Meanwhile, new DynaCab technology makes tone sculpting easier than ever, eliminating the need to sift through countless individual IRs. In fact, the DynaCab system in Cab-Lab 4 offers even more resolution and flexibility than our hardware products, with 360° of placement at variable distances, the possibility of additional microphone choices, and more. Best of all, Cab-Lab is now FREE!

Freeware - IR Impulse Response Listing

https://www.kvraudio.com/forum/viewtopic.php?p=8553062#p8553062

Here’s a very long list of free reverb impulse response as well as other IR such as a guitar amp, speaker, amongst others.

Freeware - ML Sound Lab MIKKO2 Cab Sim VST Plugin

https://ml-sound-lab.com/pages/mikko2

MIKKO2 is the most sophisticated guitar cabinet simulator in the world featuring Mikko Logrén's entire cab collection. It is also a custom impulse response creation app that adds incredible tonal variety to all guitar modelers with IR support. MIKKO2 features a raw unprocessed full resolution cab simulation that is higher quality than what any current modeler supports. Get MIKKO2 FREE to get a taste with one free cab.

Freeware - Chowdhury DSP BYOD VST Plugin

https://chowdsp.com/products.html#byod

Build-Your-Own-Distortion (BYOD) is an audio plugin that allows the user to create custom guitar effects, with a focus on guitar distortion. The plugin contains emulations of guitar distortion and tone-shaping circuits from various, along with a handful of other useful effects. BYOD supports the following desktop plugin formats: VST3, AU, CLAP, AAX, Standalone, and LV2, as well as Standalone and AUv3 on iOS devices.

Freeware - IK Multimedia TONEX CS Free Edition

https://www.ikmultimedia.com/products/tonex/

 20 included tone models for free. 10 amps, 5 pedals. 20 User Tone Model downloads.

Freeware - Overloud Blackstar Studio IR Collection

https://overloud.com/products/blackstar-studio

The Blackstar Recording Studio is a studio equipped with lots of Vintage gear and located in Milan. The main recording room as well as all the studio rooms have been carefully sampled by Valerio Mina with several microphone setups, in order to capture all the nuances and sound perspectives, for a total of 40 IRs. The IRs are designed for the REmatrix multilayer convolver, whose Player version can be downloaded for free.

Freeware - ML Sound Lab 5034 Fluff Free VST Plugin

https://ml-sound-lab.com/pages/amped-roots

Amped Roots Free is our first amp sim release and it comes with a free amp. We want everyone to experience our amp modeling quality and plug'n'play design. Just like with real amps you will not spend a lot of time tweaking. This plugin is made in collaboration with our good friend Ryan "Fluff" Bruce and the sims featured are the best amps from Fluff's personal collection.

5034 Fluff (Free) | Modern metal chugs. No overdrive needed.

Freeware - Flos Audio IR Cabinet Plugins

https://www.kvraudio.com/developer/flos-audio-plugins

3 separate Guitar Cab IR sets in plugin form.

Freeware - Audio Media Research ReValver 4 VST Plugin

https://www.audiomediaresearch.com/

ReValver® models guitar amplifiers, stompboxes and effects at the component level, resulting in unmatched realism, dynamics and tone. ReValver’s RIR 2 Cabinet Modeling takes convolution cabinet modeling to another level while ACT (Audio Cloning Technology)™ allows incredible instrument modeling on the input and tone matching on the output. ReValver can also create and load amp clones in our ACT Combo module, making ReValver virtually unlimited. Expand ReValver even further by hosting 3rd party VST/AU plugins within the software. Take ReValver to the live stage with full MIDI control and GIG Mode for seamless switching between presets. Download ReValver 4 for free and then browse the Amp Store for a wealth of additional amp/stomp/effects modules, ACT content and bundles. ReValver is available as a stand alone program or as an AAX, VST or AU plug in.

Freeware - Viator Diode Amplifier VST Plugin

https://www.patreon.com/posts/diode-amplifier-52038605

Simple amplifier.

Freeware - Studio Nord Bremen SNB Impulse Responses

http://www.grgr.de/IR/

Various guitar cabinet impulse response files.

Freeware - ML Sound Lab Amped Stevie T Free VST Plugin

https://ml-sound-lab.com/collections/amp-sims/products/amped-stevie-t-free

"Amped Stevie T Free" comes with one free amp called "Djent God". This is a custom amp sim designed by ML Sound Lab. The cabinet section includes four amazing options: ML Mega Djent, ML Mega OS, ML Mars PR-M75 and ML Bgnr Uber. This plugin is made in collaboration with the one and only Steve "Stevie T" Terreberry.

Other great amps available, also.

Freeware - Metallica Gold IR Pack

http://www.richardcookson.co.uk/Tattva/

Freeware - ZETA Guitar Cabinet IR

https://laptopguitarist.com/2019/03/28/free-zeta-guitar-cabinet-impulses/

Cabinets included: 

 Orange PPC 212 Closed Back 

Bogner 212 CB Closed Back Stack 

Diezel 412 FC (V30) 

Mesa 412 Recto Slant 

Bogner Uberkab 412 STU 

Cabs were profiled using a Peavey 5150 power amp, Shure SM57 on a Dynamount, CAPI VP28 preamp (a clone of a vintage API) and an Auralex pad under the cabs.

Freeware - Soundwoofer Top 20 Impulse Responses

https://www.soundwoofer.se/blog/about/top-20-list/