Did you know that JR EAST launched the free JR EAST Wi-Fi service in Tokyo area since October 2012?
For the first connection you have to register your e-mail address. Afterwards you only need to enter your name and e-mail address on the entry page and can use the free service for up to three hours. Then you may re-enter again.
The service is available at major JR EAST stations with Wi-Fi marks. Those include the biggest JR stations in Tokyo and airports, such as Tokyo, Hamamatsucho, Shibuya, Shinjuku and Ueno Stations. You also can use free Wi-Fi at the JR EAST Travel Service Centers at both terminals of Narita Airport, the international terminal of Haneda Airport.
For more detailed information about areas and Wi-Fi settings please take a look to this PDF.