A person who chooses to receive the benefits of a "public good" or a "positive externality" without contributing to paying the costs of producing those benefits.
(Johnson, Paul. Glossary of Political Economy Terms. http://www.auburn.edu/~johnspm/gloss/)
The collective action problem created by the existence of free riders is often called the "free rider problem." The possibility of some (or most) individuals is one of the most important obstacles to effective collective action even when collective action would benefit all of the individuals in a group.
The Stanford Encyclopaedia of Philosophy provides the following description of the free rider problem, which includes a helpful example:
"In many contexts, all of the individual members of a group can benefit from the efforts of each member and all can benefit substantially from collective action. For example, if each of us pollutes less by paying a bit extra for our cars, we all benefit from the reduction of harmful gases in the air we breathe and even in the reduced harm to the ozone layer that protects us against exposure to carcinogenic ultraviolet radiation (although those with fair skin benefit far more from the latter than do those with dark skin). If all of us or some subgroup of us prefer the state of affairs in which we each pay this bit over the state of affairs in which we do not, then the provision of cleaner air is a collective good for us. (If it costs more than it is worth to us, then its provision is not a collective good for us.)
Unfortunately, my polluting less does not matter enough for anyone -- especially me -- to notice. Therefore, I may not contribute my share toward not fouling the atmosphere. I may be a free rider on the beneficial actions of others. This is a compelling instance of the logic of collective action, an instance of such grave import that we pass laws to regulate the behaviour of individuals to force them to pollute less."
Hardin, Russell. "Free Rider Problem." Stanford Encyclopedia of Philosophy. Last modified May 21, 2003. http://plato.stanford.edu/entries/free-rider/.