The first 10 volumes of the Séminaire Bourbaki are now freely available online at NUMDAM. Until recently, only later volumes were available for free, and the earlier ones were subscriber-only. These 10 volumes cover the years 1948-1968, so they contain a lot of great material from the history of commutative algebra and algebraic geometry.
My personal favorites: Hirzebruch’s exposition of Mumford’s theorem and Grothendieck on formal schemes. What are your hidden gems from these seminars? Leave a comment!