Stronger (what doesnt kill you) leaked a long time before release and became one of the biggest Kelly's hits

In Turkey too even though it wasn't released officially at the time

I guess radio played the demo a lot

Justin Timberlake - "Magic" (so much speculation that it was going to be his comeback single)
Ciara's first (and, so far, last) solo top 10 since "Get Up"…

It was leaked for so long and it still became a hit!

Music leaked in like May 2000 and didn't get released till months and months later

Bad Romance
Alejandro

Yoü and I

Nicki's Your Love is the best example. Someone leaked it and it was mixed terribly and radio started to play it on its own which made her re-record the song and it became a Platinum hit that changed the direction of her debut album's sound, and some would say the rest of her career.

Without Your Love, there would be no Pink Friday. Without PF (RTM, M4L, SB -Taylor gets credit for this as well), there would be no David Guetta. Not Starships. Basically her career wouldn't be like it is today. PF would've came out sounding like Massive Attack, Muny, Blow Ya Mind, etc....