I like instrumentals. They're what ties everything together. Imagine your favorite song without that say, guitar piece. Not saying that a capella's are bad, but in the actual song the background music just helps with the impact.

Some instrumentals:

Don't Trust Me by 3OH!3- Well, I haven't actually found one that's completely an instrumental, but I love the guitar-ishy parts... Amazing, lol...

Love the Way You Lie- What? I listen to modern music XD Also, the piano version by Ray Mak is amazing!! Espesh part 2!!

Dark Horse- Not the main beat (Meh), more the, say, violin- sounding parts.

Okay there are waaay to many to put here (Heh heh...) Also, the Karaoke versions usually are also instrumentals. Hey, I should try that...