T-Mobile's web site. This time it's a $50 refill PIN card with the purchase of most new (not refurbished) smartphones at t-mobile.com. Currently the following phones are available with the free refill.
T-Mobile (Huawei) Prism Android phone $149.99
Samsung Dart Android phone $149.99
Samsung Exhibit 4G Android phone $179.99 (Out of Stock)
Sansung Galaxy S 4G Android phone $219.99
Nokia Lumia 710 Windows Phone $249.99
T-Mobile (LG) myTouch Q Android QWERTY slider $249.99
Samsung Galaxy S Blaze 4GAndroid phone $299.99
T-Mobile (Samsung) Sidekick 4G Android phone QWERTY slider $329.99 (Out of Stock)
All these phones as well as the refill card can be used with any T-Mobile prepaid plan, including pay as you go and the online exclusive $30/month 100 minutes, unlimited messaging and unlimited data (throttled to 60 Kbps after 5GB).
The sale ends Sept 30th but some these phones are likely to sell out quickly.