What are the various free / open source test platforms to practice ethical hacking / penetration testing?

1. Website Security Testing:

PortSwigger Labs,

DVWA,BWAPP,OWASP Mutillidae,OWASP Web Goat, OWASP Juice Shop, OWASP Broken Web application

Zero Bank, Altoro Mutual, Acunetix Vulnweb

2. CTF Websites:

Try Hackme, Hackthebox, PicoCTF, Hacker101,Vulnhub,Pentesterlab,CTFTime

3. Android Testing: 

Insecure Bank, Injured Android, DIVA, Oversecured app,InsecureShop

Helpful resources:

WebSecurity:

https://portswigger.net/web-security/all-labs

https://github.com/digininja/DVWA

http://www.itsecgames.com/

https://github.com/webpwnized/mutillidae

https://github.com/WebGoat/WebGoat

https://owasp.org/www-project-juice-shop/

https://sourceforge.net/projects/owaspbwa/

http://zero.webappsecurity.com/

http://testfire.net/

http://www.vulnweb.com/

CTF Websites:

https://tryhackme.com/

https://www.hackthebox.com/

https://picoctf.org/

https://ctf.hacker101.com/

https://www.vulnhub.com/

https://pentesterlab.com/exercises

https://ctftime.org/

Android Testing:

https://github.com/dineshshetty/Android-InsecureBankv2

https://github.com/B3nac/InjuredAndroid

https://github.com/payatu/diva-android

https://github.com/oversecured/ovaa

https://www.insecureshopapp.com/