Nickel Creek - This is a band someone showed me a while ago. They incorporate a bluegrass-esque quality to their music, which has been known to stretch into the rock, pop, and country genres.
Suggested songs: "Spit on a Stranger", "Smoothie Song" (A purely instrumental song), "Toxic" (A Britney Spears cover. )
Oh yes! Also check out their songs "Seven Wonders", "Green and Gray", "Brand New Sidewalk", "When In Rome", "The Lighthouse's Tale", "Sweet Afton", and "The Fox".
Rumor has it that when they first hit the bluegrass scene, many of the old timers were very impressed by the mandolin work done by Chris Thile.
Very good choice to add to this thread!