Music's certainly seen its share of odd-couple collaborators, from David Bowie and Bing Crosby to Nick Cave and Kylie Minogue. No pairing has felt so strangely natural, however, or been quite so successful, as the one of Led Zeppelin frontman Robert Plant and bluegrass icon Alison Krauss. Having met while performing together at a Leadbelly tribute concert back in 2004, the duo cut the startlingly good Raising Sand last year. Their interpretations of songs by Tom Waits, the Everly Brothers, Townes Van Zandt and others are moody, nocturnal, often rather haunting, and made especially captivating by their uncanny vocal chemistry. Live, they've been reshaping some Zeppelin tunes ("Black Dog," "When the Levee Breaks," "The Battle of Evermore") to fit that vibe; although those songs came out the same year Krauss was born, she and Plant perform them, and the rest of the set, like they've been singing together for decades.