Kanye West - Lost In The World, He announced it as a single after All Of The Lights and Otis but it got brushed aside because of Watch The Throne.
Kanye West - Blood On The Leaves
Kanye West - Robocop (Not sure about this one, but there is rumour that a video was filmed for it)
Katy Perry - Dressin' Up, Peacock
Madonna - Amazing - Warner Bros. wanted it it as the 4th single, but she wanted Impressive Instant. They even planned to do a live music video, so she pulled it from her setlist, so the whole 4th single was scrapped.
Madonna - I Want You - Her cover of Marvin Gaye's on Something To Remember, was supposed to be the lead single but was cancelled due to contract problems with Motown Records, so You'll See was released instead
Fergie - Finally, I guess wasn't TECHNICALLY cancelled, but the video didn't get released
Jennifer Lopez - Brave
JoJo - The Other Chick, Demonstrate

Damn Blackground