Smartphones can be used as a wifi router to share your (Wifi), 3G, 4G, Internet connection over WiFi, USB, and Bluetooth.
The main difference between a wifi router (like the MIFI) and a smartphone used as a router is that you can use the smartphone as "WIFI to WIFI" bridge, where the mifi bridges only from "3G to WIFI".
It means you can connect your smartphone to a WIFI hotspot (Mc Donalds, hotels, restaurants,...) with web authentification and broadcast a wifi compatible with a WT or WFT.
Use PDANET to tether your internet connection.
A Wt5 alternative (for D4 only)
Other useful app: