Kristy Lee Cook, left, and Jason Castro are seen backstage at the "Idol Gives Back" fundraising special of "American Idol" in Los Angeles on Sunday April 6, 2008. (AP Photo/Matt Sayles)

Maybe it's that image of carefree bluebirds or just the lilting lullaby tone, but there's something about "Over the Rainbow" that warms the heart and makes cash registers ring.

The latest star turn for the tune written for Judy Garland's Dorothy in 1939's "The Wizard of Oz" came last week when dreadlocked "American Idol" finalist Jason Castro plinked out the song on a ukulele.

Castro's rendition not only kept him in the competition, it also spawned a 700 percent increase in sales and a huge jump (from No. 187 to No. 11) on Nielsen SoundScan's digital songs chart for the 1993 version recorded by the late Hawaiian giant Bruddah Iz, aka Israel Kamakawiwo'ole.

Other "Idol" performances also drove download debuts: Martina McBride's "Anyway" (No. 101) and Aerosmith's "Dream On" (102), sung on the show by Kristy Lee Cook and Michael Johns, plus 10 songs performed on "Idol Gives Back."

"The reason for (the song's) enduring success is simple: It's a great song with an inspirational message that can be applied to any situation," says Rolling Stone managing editor Will Dana. "I'd put it up there among the five most well-known songs in our culture."

Iz's sparse interpretation has starred in countless movies, TV shows and commercials, include a 1999 ad campaign for eToys.com and a 2002 episode of "ER" focused on the death of Mark Greene. But the singer, who battled obesity, didn't live long enough to see his vision of Garland's classic resonate so completely with the masses. He died in 1997 at age 38.

Castro's decision to sing Iz's version of "Rainbow" may have been as inevitable as it was deliberate, says Dana. Garland's version "came out a long time ago, and it's possible that (all) today's kids know is the Hawaiian interpretation. Either way, (Castro) did well. On 'Idol," it never hurts to be unique."

Another music industry veteran ascribes the longevity of "Rainbow" to the magic that happens when the right words are mated to the perfect melody.

The Beatles' "Yesterday" "is another one that is just so timeless," says consultant Tom Vickers, a former music publishing executive. " 'Over the Rainbow' seems to have new lives with each generation, whether that's fresh takes by (guitarist) Tommy Emmanuel or Israel.

"What you hear in Iz's voice is up to you, whether it's the search for that pot of gold or a person to complete your life. In these tough times, a song like that will resonate."