In January, on the steps of the Lincoln Memorial, Pete Seeger was the oldest person to perform as part of Barack Obama's inauguration festivities. 

Singing the "greatest song about America ever written" (Bruce Springsteen's words) before 500,000 people live and tens of millions more on television, the then-89-year old legend crooned two little-known verses of his friend Woody Guthrie's 1940 patriotic standard, "This Land is Your Land" -- both about Depression-era poverty -- restoring the song to its former glory over the sanitized version that ruled for too many years.

Over the course of a remarkable lifetime, Seeger has been an ambassador for peace, social justice and the best kind of patriotism. A uniquely American mix of blueblood and bluegrass -- a product of Harvard University and the son of a violinist mother and musicologist father -- Seeger has lived the story of the American left in the 20th century. The celebrations of his 90th birthday on Sunday offer a good opportunity to showcase and celebrate the causes to which he's devoted his great life.

Defiantly leftist, pacifist--and for a decade or so, Communist--Seeger has embraced and supported virtually every major progressive advance of the 20th century. He's sung and spoken out for organized labor, against McCarthyism, in support of racial justice, on behalf of nuclear abolition and against the Vietnam War; his voice put early wind into the sails of the environmental movement.

The right to dissent in a democracy has been a cornerstone of Seeger's activism. In the fourth episode of the video series This Brave Nation Seeger talked about the infamous 1949 riot in Peekskill, NY, and the impact it made on his political development and commitment to free speech.

Seeger's songs have engaged people, particularly the youth, to question the value of war, to ban nuclear weapons, to work for international solidarity and against racism wherever it is practiced, and to assume ecological responsibility.

A particular hero to the civil rights movement on whose behalf he's worked so tirelessly, Seeger made his first trip south at the invitation of Dr. Martin Luther King, Jr. in 1956, and returned in '65, again at King's personal invitation, to join the march from Selma to Montgomery, Alabama. Amid the tension and heat, Seeger went from campfire to campfire when the marchers stopped for the night, raising morale with rollicking sing-alongs of new freedom songs.

Seeger also vigorously joined protests against the Vietnam war, playing countless benefits and protests and recording "Waist Deep in the Big Muddy," the lyrics of which have renewed relevance today: "But every time I read the papers/That old feeling comes on/We're waist deep in the Big Muddy/And the big fool says push on."

Sometime soon after King's assassination in 1968, Seeger began to focus his energies locally around the town of Beacon, New York and the notoriously polluted Hudson River. Gathering together friends and colleagues, he picked up a literal hammer, this time to build the sort of sailing ship that hadn't been seen on the river in decades to raise consciousness of environmental issues. They named it the Clearwater. Seeger also established Hudson River Sloop Clearwater, a group which sponsors annual eco-festivals and acts as a bulwark against polluters in the area. Today, people can swim in the Hudson again.

Seeger birthed a folk revival that remains strong and relevant, and the music he championed is still sung on marches and picket lines coast to coast. As he moves into his tenth decade, it's worth celebrating the music he has made--and the changes he has helped to bring about.