Stack Overflow is the most popular site to go to if you have questions or problems about programming. The community is incredibly active and helps solve programmatic headaches every day.
Cross-Validated, or “Stats Overflow” is an exchange part of Stack Overflow that is focused on statistics and the more statistical aspects of machine learning.
Data Science Overflow
Data Science Overflow is an exchange part of Stack Overflow that is focused on data science and machine learning.
Kaggle is a very popular platform which contains datasets, competitions, as well as discussions about machine learning.