www.timeout.com/london/
THE magazine to read what's going on. While I like zitty in Berlin much better, you can not get around TimeOut London in this city.
http://flavorpill.com/london
mainly an event page for affordable events. they announce good stuff. unfortunately often only the day they are happening.
http://gridskipper.com/travel/london/
very good thematic maps.
http://londonist.com/
site that talks about every important and unimportant thing that is going on in this city. concert reviews, more or less serious political reports. it's fun to read.