From Transport for London (TfL).

The London Tube now has WiFi at more than 100 of their stations thanks to Virgin Media. Depending on your service provider (boy does that sound like commercial-speak or what?), you will be able to use the system for free. Luckily for me, that includes Vodafone, which is the carrier for my UK mobile.

Here is a map of WiFi-enabled stations. A couple of biggies are in the works but not yet ready, such as Heathrow Terminal 5, Earl’s Court, Bank and Sloane Square (oh those poor Rangers!). The concentration appears to be the zones 1 and 2, so the tourists will be pretty well taken care of. Looks like DLR and London Overground will have to wait.