Physics.Stackexchange - The largest question and answer physics community.

PhysicsOverflow - An alternative to Physics.Stackexchange, which was started by a group of users unhappy with the restrictive moderation.

Physics Forums - The largest physics forum.

Quora Physics - Another question and answer community, which is especially good for meta questions and opinion questions, which are not allowed at Stackexchange.

/r/Physics/ - The physics subreddits mostly focuses on discussions of recent news.

CosmoCoffee - Another nice physics forum with a focus on cosmology.

sci.physics - The oldest physics community, which is however no longer very active. However many great discussions can be found in the archives.