There are many methods of Free shipping as there are many reasons you may want to use them ...
Modules ... Order Total ... Shipping ot_shipping ...
Let's say you want nothing but Free Shipping when the Order reaches $75.00 for National (matches the Store Country) ... or International (any country not matching the Store Country) ... or Both ...
NOTE: this turns off all shipping modules when it displays ...
Modules ... Shipping ... FREE SHIPPING! freeshipper ...
Let's say you want to offer Free Shipping on a Product with 0 weight (when 0 weight is set to mean Free Shipping in the Configuration ... Shipping/Packaging) ... or when Products are marked as Always Free Shipping ... (note Always Free Shipping means for "Everyone") ...
NOTE: no other shipping module shows when this shipping module shows as it is triggered by things that turn OFF the other shipping modules, 0 weight or Always Free Shipping ...
Modules ... Shipping ... Free Shipping Options freeoptions ...
Let's say you want to offer regular shipping modules, such as UPS, USPS, etc. but you also want to offer a Free Shipping Option ... you could have this one show with the others "all of the time" by setting the Total >= 0 ... or you could set a condition such as Total >= 25.00 ... or Weigh <= 12 ...
Then, of course, you can customize all of these modules further for other conditions for when they run or don't run as well in the code ...