FreeSound is one of the first collaborative website that allow anyone to upload their sounds and share them for free to anyone. It's one of the biggest source for free sound any where on the net. It has a huge community, and the outreach is huge. Although all sounds are free to download, they have different licence depending on the creator preference. It can be not suited for commercial work, or you would need permission from the author, just be sure to check out the detail of the licence.
16000 sounds were made available to download from the BBC archives! Though most of these sounds are a little bit old, they still are useful and diverse. However the licence right is limited to non-commercial use (see details of the licence there: RemArc licence).
Check it out: http://bbcsfx.acropolis.org.uk/
Zapsplat is one of the most consistent and quality-oriented free sound library. With over 30000 from various professional contributors, it is absolutely one of the best platform to check out
Airborne Sound has one of the best offer of sound effect online. They kindly give some of their sounds for free. Paul Virostek gives so much to the sound effect/ Field recordist community. Checkout their free section which is well furnished.
Yes, the institution of NASA released incredible sounds they recorded in space or on the ground. They made them available to download on their website! (it is in mp3 format and is not royalty-free but you can use it for non-commercial use).
Check it out: https://www.nasa.gov/connect/sounds/index.html
Since 1998, Franck Bry alias 'The Recordist' has done an amazing job in creating impressive and useful sound libraries. He has a free section which contains mp3 version "Free for personal use only". Check it Out: http://creativesounddesign.com/the-recordist-free-sound-effects/
Bonson from Michel Marchand is creating is creating some of the most compelling and innovative sound libraries out there. On the website, you can get a free pack of sounds from Bonson, you just have to fill in you email address.
Check it Out: https://www.bonson.ca/product/free/
Articulated Free Sounds
On our own website, you can find a section of high-quality free sound libraries usable for commercial work, to get them, it is simple, you just add them into your cart and do the checkout process, then you'll get the download links.
Check it out: https://articulatedsounds.com/audio-product/sfx/free-sound-effects
SoundMorph is a contemporary sound effects company, they also propose software tools. Very nice offer, and they do have small free sound library for you to download. Check it out: https://www.soundmorph.com/sounds
Boom Library is probably the most well-regarded sound library company. They have been leading the way in creating high quality sound packs. They have some free sounds on their website, but also through their email list.
Check it out: https://www.boomlibrary.com/
99 Sounds gather individual libraries made by sound designers of all shades that seek to promote their work. Some libraries available on their interface have top-notch quality. The sounds are usually in high definition and royalty-free. A must to check ressource!
Free to Use Sounds
Free to Use Sounds is the endeavour of Marcel Gnauk, which is prolific in recording sounds from many countries, check out his work
The Internet Archive is a non-profit organization that seek to provide universal access to all knowledge. It has all kind of digital media, and section dedicated to audio. With most of the sound under creative commons, it has over 4 millions sounds & musics available. You can search by category, key-words, and tags. A precious resources.
Check it out: https://archive.org/details/audio