Amortentia

Amortentia is the most powerful love potion in the world. It causes a powerful infatuation or obsession in the drinker. It has a distinctive mother-of-pearl sheen, and steam rises from it in characteristic spirals.

It has a different aroma for everyone who smells it, reminding each person of the things that they find most attractive e.g. Hermione Granger smells fresh cut grass, new parchment and Ron Weasley's hair (the latest wasn't divulged in the book, but J.K. Rowling revealed it in a webchat), Harry Potter smells treacle tart, a broomstick, and "something flowery that he thought he might have smelled at the Burrow" (which he later realises is the scent of Ginny Weasley).

The effects of the potion on a person are described to be near instantaneous; with the person who drank the potion appearing "pale and sickly", and becoming dangerous obsessed with their object of their affections, talking about them looking "as though hit by a ray of purest sunlight", and becoming excited or dangerously unstable towards others.

Known Uses

 * Merope Gaunt, who cherished a burning desire for Tom Riddle Sr, used her re-surfaced magical abilities to create potion, and eventually convinced Tom to take a drink of water spiked with the potion on a hot day riding alone on his horse. This had the desired affect as Tom, now obsessed with Merope, married her and they ran away together. When Merope became pregnant with Tom's baby, Merope, believing Tom would willingly stay for the baby's sake, or unable to continue to enslave him by magical means, stopped giving him the potion.
 * Romilda Vane, a romantic admirer of Harry who saught to make him invite her to Professor Slughorn's Christmas party, gave Harry Gillywater and Chocolate Cauldrons both spiked with potion. Harry was forced to take the latter, though simply abandoned them in his trunk for months until they were thrown out during Harry's search for the Marauder's Map and, believing they were a birthday present, Ron consumed them. Harry promptly took Ron to Slughorn, who gave him an antidote.

Behind the scenes
It was stated by J.K. Rowling that a large fountain or well filled with Amortentia could be found in the love room in the Department of Mysteries.

Appearances

 * Harry Potter and the Half-Blood Prince