Which DROID are you talking about? Certain ones can be used on T-Mobile as they do use a Sim card and also have GSM/UMTS radios in them. However if you are talking about the Motorola device that was released in 2008(I believe) then it will not work.
Outside of the Iphone, Verizon phones use the CDMA network. This means that they do not use sim cards and there is no place to insert a sim card. If you are looking for an unlocked Droid phone,I would look at a AT&T unlocked device.
I have an unlocked Verizon Droid DNA and I am using it with my T-Mobile SIM card. I can only get EDGE signal and some of my features don't work without a Verizon SIM card. So if u receive the phone for free then it's worth a try.