Chess.com. They have a nice iOS app too for playing the games on the go.

I am not sure what specific activity you are asking for. There are play sites, news sites, forums, learning sites etc. I like chessbase for news. But I play at fics which is a chess server.

I use gameknot.com. Good for correspondence style chess games. Lots of people too.

redhotpawn.com - good for correspondence chess and an active chess forum.

chesscafe.com - a great collection of articles (including instruction) on chess

chesstempo.com, for tactics problems, is the chess site I visit most frequently.