In Ser Duncan the Tall's entry in The Book of Brothers it mentions that he named a commoner as the Queen of Love and Beauty after winning a tournament at Pennytree which King Aegon V Targaryen held in his honor.
Sansa Stark visits the statue of her aunt Lyanna Stark in the crypt at Winterfell, lighting a candle in the statue's hand. Sansa is joined by Petyr Baelish, her recently widowed uncle by marriage, who recalls what happened at the Tourney of Harrenhal where Rhaegar Targaryen shocked everyone present by passing over his wife in order to crown Lyanna as the Queen of Love and Beauty. Baelish concludes his story by noting how this seemingly sweet gesture led to deaths of thousands.
In the books
In the A Song of Ice and Fire novels, the tradition holds that the victor in a tournament may select any woman present and name her the queen of love and beauty, crowning her with a wreath of flowers and dedicating his victory to her.
The victorious knight often chooses a woman he loves or intends to court, and it can be a source of scandal if the victor crowns a woman already bound to another man, or if a married man crowns someone other than his wife. Since both of these cases were true when Prince Rhaegar Targaryen crowned Lyanna Stark after winning a tourney at Harrenhal, it was particularly scandalous.
Queen Naerys Targaryen was crowned by her brother, Prince Aemon the Dragonknight, who was disguised as a mystery knight known as the "Knight of Tears". Queen Naerys was married to King Aegon IV Targaryen.