You have most probably used 7805 voltage regulator many times. It is a simple way to get regulated +5V power supply.
A Detailed Look at the 7805 Voltage Regulator post at hackaday.com points to an an amazingly detailed article about its theory of operation and implementation of 7805 regulator. As some elements looked a bit odd during [Ken]‘s reverse engineering process, he finally concluded that what he purchased on Ebay may be a counterfeit (read this Reddit comment for another opinion). That’s why the article was named Reverse engineering a counterfeit 7805 voltage regulator. Here is one picture from that article: