The Samsung Galaxy S 4G is only free until later today, so go get one now if you haven't done so already. You have a bit more time, until this Thursday, to get a myTouch 4G for free though.
This offer is online-only, so don't bother going to a TMo store and asking for your free smartphone. Just go to T-Mobile's online shop and get yours.
The Samsung Galaxy S 4G has a 4-inch 480×800 capacitive touchscreen, a 1 GHz processor, 512 MB of RAM, 1 GB of ROM, a 5 MP autofocus camera with 720p HD video recording, 21 Mbps HSDPA, Wi-Fi, Bluetooth, GPS, and a 1650 mAh battery.
The T-Mobile myTouch 4G has a 3.8-inch 480×800 capacitive touchscreen, a 1 GHz processor, 768 MB of RAM, 4 GB of internal storage space, microSD card support, a 5 MP autofocus camera with LED flash and 720p HD video recording, 14.4 Mbps HSDPA, Wi-Fi, Bluetooth, GPS, and a 1400 mAh battery.
source: Unwired View