I'd have to recommend
http://www.net-chess.com, but only because I wrote it.
But seriously on improving your chess. I think "how" you improve your chess play greatly depends on "why" you want to improve your play. Very few people actually plan to become professionals, so it does not make sense to try to copy their learning strategy, and in fact, a professional player probably isn't even good coach for the "casual but still serious" player.
I have been playing in the "serious but casual" arena for about 20 years now, the highest USCF rating I've achieved was about 1750. I used to be very active in the local chess community being cheif TD of at least one USCF rated tournament a week, sometimes 2 or three, and would play in about as many. I had seen many people start well below my rating and excel past me, and many stay well behind. I'd say the main difference between the 1800+ and the lower players is the amount of time they spend studying openings, games resulting from specific openings, and specific endgames. I've tried to study such things, but honestly every time I've tried I ended up reminding myself that I'm mostly playing for fun, and there is not point in torturing myself for something I just do for fun. So, if you actually enjoy sitting down and reading random books on theory, that's probably the way for you to go. But just because you don't want to do that, know there's a lot of fun to be had just playing people your own skill level, and you will (slowly) improve.
On a side note, there's some players who learn all the ins and outs of one archaic/unknown opening and use it all the time. These people usually rise to a certain level and platou, they might break 1600, or even 1800, but never 2000 without a significant change in strategy. The reason those openings are unknown is because they're usually weak. And a good player will take a slight advantage in the opening and leverage it for a win. At lower levels, a slight advantage in the opening usually isn't game determining.