1st provider: proxy-sale.com

Go to https://proxy-sale.com/ and then choose Free from the main menu:

Features

1. Free Proxy pool is updated regularly.
2. Includes Anonymous proxy
3. Includes SOCKS 4/5 proxy as well as HTTP/HTTPS ones

2nd provider: proxyscrape.com

Another free proxy list updated every 5 minutes.
Useful feature: One may get updated free proxy list thru API call: api.proxyscrape.com/v3/free-proxy-list/get?request=displayproxies