...you may be familiar with the phrase "common law marriage."  It was applied to couples who lived together for a certain period of time.  Their "marriage" would be recognized as legal even though they didn't have a license.

With so many laws today covering everything, there aren't many common law situations recognized today.  In California, you can still change your name by common law without going to court to officially change your name.  You just use whatever name you want, but you run into problems with anyone asking for ID.  Try getting the DMV to give you a new licence with the name Big Papasan just because that's what you want to call yourself.

as a general proposition as opposed to law codified in statutes.

        When the Supreme Court issues an opinion, that is common law of the highest court in the land and is binding on all federal courts deciding federal or US constitutional questions. But all states have courts that issue judicial opinions so every state is a common law state, except perhaps for Louisiana which is civil law state.

       While most common law rules are similar in every state, sometimes there are differences. States that are commonwealths like Massachusetts still have courts that create and apply common law, so there is no distinction between a commonwealth and a state as far as the common law is concerned.