Where are Serre’s lectures at Collège de France to be found? Having run into several references, at various places and occasions, to "Serre’s Course at Collège de France, 19xy-19xy+1" for various values of xy, I would genuinely like to know where these lectures are written down and archived. 

Has there been an effort to preserve them in any form? 

There are overviews of them in Serre’s collected work, but they have no references and don’t give full proofs.
 A: The detailed abstracts (résumés détaillés) that are referenced are supposedly available in the "Annuaire du Collège de France". The journal's website unfortunately only contains newer entries (2005–). However, your library might have paper versions. According to Mathdoc, some physical versions are available in a few French libraries (notably at the IHP, the ENS, at the CIRM – with a few gaps in each case, unfortunately).
Some of the published versions that Carlo Beenakker mentions are available at Numdam. You can directly search for Jean-Pierre Serre's works. It might take some digging to know which lecture corresponds to which published paper, but the titles would probably help.
A: Most of my lectures have been taken down (hand written) and copies are available from Inst. Henri Poincaré. See Numdam CJP Serre.
JPS
A: The lecture notes for many of Serre's courses have been published:


*

*Groupes algébriques et corps de classes (transl. Algebraic Groups and Class Fields) (Course at Collège de France 1956-1957)

*Algèbre Locale, Multiplicités (Course at Collège de France 1957-1958)

*Corps locaux (transl. Local Fields) (Course at Collège de France 1958-1959)

*Groupes proalgébriques (Course at Collège de France 1959-1960)

*Cohomologie galoisienne (transl. Galois cohomology) (Course at Collège de France 1962-1963)

*Arbres, amalgames (transl. Trees) (Course at Collège de France 1968-1969)

*Lectures on the Mordell-Weil theorem (Course at Collège de France
1980-1981)

*Représentations linéaires des groupes finis (Course at École Normale
1966)

*Cours d'arithmétique (transl. A course in arithmetic) (Course at École Normale 1962-1964)

*Groupes finis (transl. Finite groups) (Course at ENSJF, 1978-1979)

*Topics in Galois theory (Course at Harvard University 1988)
