I have resurrected this sticky. I know it's target is PHP at the moment, but I'm planning to broaden the subject for web development (HTML/CSS/Js).
If anyone has something useful to add, please let me know in this thread and I'll look into adding it. Do NOT just reply with links, please provide a reason for adding. Link only replies will be ignored. Use the comment/voting system if you (dis)agree with someone's addition, I will take that into account.
Please keep it pleasant.