1) The Qt project and it's very great and well written documentation, it helped me to learn english back in that time.
2) The SFML project, it was making it so simple to create 2D games with C++ compared to what i knew in C with SDL...
2) The Django framework, i was coming from a Symfony 2 + 3 with a bit of knowledge in Java (JSF, JSP, etc...) but when i discover the Django framework with his batteries already included, those was real talking about stuff like :
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