Can anyone tell which is the best site of open projects for beginners?

Answer 1

sourceforge.net ;)

Answer 2

www.github.com ;)

Answer 3

do you mean open project cloudly or finding the project ?
there is a lot in the internet,you can find anything you want, but I give you some recomendation:

1. freecodecamp.com
this is interactively to learn from front-end development, data-vis development and back-end development

2. Quora.com
you can find any answer from your question there

3. Github.com
officially this site is call "social coding" so you can share your experiment with the others

4. upwork.com
if you interested find more difficult project on the marketplace freelancer one of the most popular is upwork.com

good luck !