Do you guys know some site that is really good, well organised, with video lectures for learning general concepts of chess like openings, middle-game, end-games, tactics, and, furthermore, for free? What do you recommend? Grateful!
It's hard to make a suggestion without knowing your rating of approx. level, but here's a try.
My suggestions would be the following YouTube channels:
(1) Majnu. Especially check out the "Chess middlegame training" and "Practical chess endgames" playlists. I like Majnu's videos because, not only is he a good player, but he also manages to explain ideas and plans in chess in a very simple manner. The concepts really stick in your mind, and you get to actually use them in your games. And from the way he explains things, you can see that he loves the game, which is also inspiring. I recommend Majnu highly (especially to beginners).
(2) SeanGGodley. Excellent analysis of famous games and players. The book "My system" by A. Nimzowitsch is considered to be one of the most important books in chess history, and Sean has a playlist dedicated to this book. Excellent and (pretty) detailed analysis make him one of my favourite YouTube channels for improving my game. Also, Sean is a great narrator in my opinion, and that makes it easier to follow his thoughts. However, Sean's videos are maybe too much for beginners.
(3) thechesswebsite. Many videos on openings and traps, as well as other things. Just check out the playlists on this channel. Great stuff!
Of course, there are more interesting channels out there, but these are my favourites. I would also like to mention KingsCrusher, who produces videos on a daily basis (for couple of years now). This channel discusses a lot of different chess topics, so it's very likely that you'll find something of interest there.
Try my blog.I am posting some postion and some videos that are made by me currently I am making videos in marathi language but hope I will be doing in english soon..The position given are very instructive..I hope you too like them the link of the site is http://koravichess.blogspot.in/