Young people today probably know little if anything about Crosby beyond his recording of White Christmas. He's been away from the scene for nearly forty years now, but that recording remains the best selling single of all time. I think he sits - with Bob Hope - at the pinnacle of the American entertainment industry in the 20th century and is well worth exploring if you enjoy popular culture. The Crosby family has authorized a comprehensive site about The Crooner if readers want more information.
Seeger was born into a musical family in New York, took up the family's leftist politics, and made a name for himself as a "protest singer" in the 1940s. In 1950, he was a member of the folk group, the Weavers, and in the bow wave of a folk music revival in the U.S. It was short-lived, however, as the group was blacklisted in 1953 for suspected political reasons.
Seeger found himself at the forefront of the 1960s folk revival as well. Over his last decades Seeger continued singing and pursuing his social, political, and environmental activism around the world. Putting his politics aside - in 2007 he announced unease with his communist past - I think Seeger is our best-remembered if not loved folk musician of the last century. Woody and Arlo Guthrie, and Bob Dylan may come close as performers but there was something about Pete Seeger's sincerity and warmth that put him at the top. He had the best voice, too! For more information and a host of links, here is his Wikipedia entry.
I can never get enough of the Weavers: Ronnie Gilbert, Lee Hays, Fred Hellerman and Pete Seeger.