A freeloader is a program that uses some system or server resources to survive and possibly benefit its creator, without paying for them. Servers may provide some minimal service for free, in order to attract paying customers, or unintentionally, as an unintended effect of complex cost
structures; there may be ways to arrange for some transaction charges, especially small ones, to be lost in the shuffle. A freeloader exploits these sorts of things to operate free of charge.
Also see: weeds, Flying Dutchmen.