There are many sites about No Limit Texas Hold'em. What are the best for learning No Limit Texas Hold'em? Please separate your recomended sites for newbie, medium and advanced level players (or write which sites are good for every level). Please suggest both free and pay sites.
Before I start, let me say that if you are from outside the US I would definitely take advantage of sites that offer rake-based-membership. In the US, we have DragTheBar, which offers free membership based on the rake taken from your hands at Carbon Poker.
I would even go a step further and say try every single site that offers a free trial. Stagger them (start a new trial as the other ends) and you'll be set for months. Just make sure to cancel before the trial expires.
Note: Because I'm a beginner, I will only offer recommendations for the beginner levels. My current game is Cash, 4-tabling 6Max 5NL. If you play Tournaments (MTT, SnG) then you'll want sites geared for those as well.
Generally, you should be playing the micros (2NL-50NL). Because of this, you'll want to focus on sites who also focus on the micros. And while there are plenty of sites that focus on beginners, you really want to focus on micros.
Training Site - Drag the Bar
If for any other reason, check Drag the Bar because BlackRain79 is a coach there. He was (is?) the #1 micro stakes player in the world in terms of ridiculous volume and profit. The coaches there are great and in my narrow experience, seem to be far more prepared than on other sites.
Community - CardsChat
As a beginner you'll be slammed with too much information anyways, so it's important that the information is as easily accessible as possible (and easy to ask). TwoPlusTwo is in the top 3 thousand sites visited in the US. That's huge, but in my opinion it's too huge. Unless I land on 2p2 through Google, it's just too intimidating for the beginner [me].
CardsChat is simple - all the top level forums can almost fit on a single screen! I'm only just now breaking out of my social-shell, but once I get a certain number of posts I'll also be eligible for their Freeroll tournaments...and you can too.
Their strategy articles are also well organized and will get you competently playing in an afternoon.
This doesn't answer your question (beginner to advance), but it was too long to leave as a comment so there you go :)
I strongly recommend reading all the Doyle Brunson articles from his website . In this wonderful collection of articles, Brunson talks about limit & no-limit hold 'em, when to call, raise and fold.
"How To Play" section of WSOP is another great resource worth spending time on.
pokerstrategy.com is GOLD. They have many articles covering different topics and parts of online poker. Also they do classes, so they stream screen of lecturer and you can directly ask the lecturer about his play. It is really best way to jump into poker right from the start.
(My referal link: http://cs.pokerstrategy.com/#referrer=asketak )
forumserver.twoplustwo.com is another great server with long history and a lot of content. It is little bit less newbie friendly. As newbie, you can not easily realize, which topics and posts relate to you and who speaks great thoughts and who speaks nonsense. As you get better at fundamentals, you can find some gems, that can push your edge. A lot of the topics here discuss really advanced concepts and so even pro players have sometimes different answers.
TwoPlusTwo poker forums. Nothing else.
I highly recommend the following collection of forum posts / articles. These are the finest writings about the basics and more advanced concepts of the game, these will put you lightyears ahead of micro / small stakes competition. I remember when i first came by this around 2008, I got so hooked :) Have fun!