Genius

New-ish from Apple. You pick a song and Genius mines your iTunes library for the top 100 songs that match it. I guess based on how other people group the song in their playlists - kinda like a nearest neighbor data-mining algorithm. Been playing with it lately. Tonight, I picked True Faith by New Order. The top 10 out of 100 that Genius matched it with:

True Faith: New Order
Policy Of Truth: Depeche Mode
Love Song: The Cure
Eyes Without A Face: Billy Idol
Rock the Casbah: The Clash
Once in a Lifetime: Talking Heads
I Melt With You: Modern English
Modern Love: David Bowie
Roam: The B-52's
Friday I'm In Love: The Cure
Missing: Everything But The Girl

I picked Many Shades of Black, Genius picked:

Many Shades Of Black: The Raconteurs
Dead Leaves and the Dirty Ground: The White Stripes
Walken: Wilco
Australia: The Shins
Incinerate: Sonic Youth
Dope Nose: Weezer
The Bends: Radiohead
Old Yellow Bricks: Arctic Monkeys
Cellphone's Dead: Beck
Slow Night, So Long: Kings Of Leon
Supermassive Black Hole: Muse