If you've been visiting some of the major chess sites you've probably come across banner and other ads for Chess24.com. This young site is clearly ambitious, and at least based on the current content I hope it succeeds.
Some bits of the site are accessible without a membership, including reports on the world championship (the round 5 report is here). They're also transmitting the games live with computer analysis and, as a nice little feature, the time used after each move. (Voila.)
Their playing zone isn't so attractive yet, in part because it's scarcely populated and there's no simple way to download your games (whether in PGN or some other format). One just selects a time control and waits for the computer to pair you, and it can be a long wait. (But that's just for now; as the site becomes better known that's bound to change.) There's also a tactics trainer feature, which is a nice aspect of the site.
The main attraction of the site are video series. The crown jewel at the moment is a 12-hour repertoire series on the Gruenfeld by none other than Peter Svidler. (There's a sample here.) No PGNs here either, alas, so one must manually enter the analysis into one's own database, but it's still worthwhile for aficianados of that opening. That, as I said, is the main attraction at the moment, but there are other attractive video series for a wide range of playing strengths. Viswanathan Anand had a few series dedicated mostly to lower-rated players, but some of the later examples in his tactics series can be enjoyed by stronger club players. There's a very good series on Magnus Carlsen by Artur Jussupow, with Jan Gustafsson in a supporting role. Gustafsson has a good series on formulating a 1.d4 repertoire, not so much delving into deep theory but offering a wide range of move order considerations. These can be very useful, and are typically underappreciated by club players.
There are other elite presenters like Rustam Kasimdzhanov and Matthew Sadler, and still more series by non-elite titled players. Not all of the series are in English - not by any means. German and Spanish-language video series are very common on the site, so if those are your native languages you're in luck. Spanish speakers benefit by the presence of a 2700(ish) commentator, too: Francisco Vallejo Pons.
The site can surely improve in various ways, but it's pretty attractive even where it is now. They're adding content pretty regularly too, so it's not as if a membership will just get you what's there now and that's the end of it. At this point I wouldn't really recommend signing up if your main aim is to find a place to play online or to look for a place to practice tactics. Far better playing zones exist and there isn't any real shortage of books and sites to practice tactics.
The salient question is whether one wants a membership to watch their videos. There are excellent video sites on the web (given that I do videos for ChessLecture.com, this claim shouldn't be surprising), but if chess24.com keeps creating video series by 2700+ players it will be hard to beat for that form of chess instruction and entertainment. (Especially for those who also understand German and/or Spanish.)