I've put together a list of some essential Seeger recordings, as found on YouTube. The Smithsonian Folkways site offers so much more to discover.
"Worried Man Blues"
Pete Seeger on the Johnny Cash show -- ends with a great duet on "Worried Man Blues" - Pete gets tha audience singing, then Johnny joins him. Incidentally, this clip includes a fragment from a song I don't think Pete ever recorded.
"Down by the Riverside"
Pete performing with Brownie McGhee and Sonny Terry.
"Michael, Row the Boat Ashore"
Recorded in concert in Melbourne in 1963.
"Which Side Are You On?"
The words are by Florence Reece; the tune is traditional. No info about when and where this was recorded; it's really an in-concert audio recording with added slides, but an excellent performance.
Pete and the Weavers performing their best-known song from their last concert in 1980.
This traditional ballad is another audio-only recording with a still photo.
"Vive La Quince Brigada"
Bonus track: There's a great version of a Yiddish song called "Hey Zhankoye" linked to from this page. Scroll to the bottom to play it. I couldn't find a more direct way to get there. It's from Pete's 1957 concert at the University of Chicago; I may have been there.