| It's not just|
a good idea
Common law is a body of English and Welsh (and later Commonwealth and U.S.) legal precedents separate from written laws, such as statutes (enacted by a legislature), regulations (promulgated by agencies), and written constitutions. Common law is also distinct from equity, a body of principles that developed in the English and Welsh courts of chancery.
Crucial to the development of common law is the princple of stare decisis, under which the decision in a case before the court is resolved based on prior judicial decisions. A good deal of American and British law remains common law, rather than legislative law, and is based mostly on judicial guidance. Common law is contrasted with civil law, the legal system most common outside the anglophone sphere, which is based entirely, rather than partially statutory law, especially the code napoléon.
 The common law and lawquackery
To a certain strain of irrational thinker, however, especially American and often espousing extreme right-anarchist, racist, or fascist viewpoints, "common law" represents a semimythical body of legal theory that represents a type of idealized state free of judicial activism; in practice, the end result is generally believed to be some kind of quasi-anarchic state with very few fetters (i.e. taxes or regulations) on the individual. Manifestations of such thinking include the militia movement and often-connected extrajudicial courts, unrecognized by any governmental authority, which invoke this concept of common law to file charges and bring legal decisions against governmental authority. Many of the "common law" advocates in the United States are disaffected rural white males, usually fundamentalist Christian in their belief, and distrustful of government due to perceived intrusions on local lifestyles.
Some tax protestors claim that the only reason we've been duped into paying income taxes is that we have allowed ourselves to be entangled in "statutory law", and that it's possible to opt out of every statute in the land -- including those statutes that impose taxes -- and be subject only to the Common Law. Their claims to be outside the jurisdiction of statutory law hold up in court every bit as well as their other pseudolegal arguments.