ZTE the leading company in 3G modems in the world. One who wants to unlock ZTE 3G MODEM MF100, MF180 AND MF190 for free can follow the guide :

1). Download DC Unlocker from the link which is provided below at the end of the article.
2). Extract it to a folder (both .exe and .dll )
3). Plug your MF100 / MF180/ MF 190 modem with an unaccepted SIM 
4). Run dccrap.exe then this window will be appear.
5). In Select manufacturer, choose ZTE datacards and choose model as _Auto detect.
6). Press on the Loop (detect card) to detect your data card.
7). To proceed to unlock your modem, In SIM Lock status must show locked.
8). Click on Unlocking then Press Unlock. ( Your modem is now unlocked)
9). Create new profile as per your network provider (Using APN and User name and password), try to connect to the internet now.