As pointed out on MathOverflow and elsewhere, all ICM proceedings up to 2006 have been made available in searchable format at the IMU website. I have started skimming through some of these and will post here direct links to the articles that seem the most interesting. From 1950, I particularly like the following:

- Zariski: The fundamental ideas of abstract algebraic geometry (pdf, djvu)
- Weil: Number theory and algebraic geometry (pdf, djvu)
- Hurewicz: Homotopy and homology (pdf, djvu)
- Eilenberg: Homotopy groups and algebraic homology theories (pdf, djvu)
- Chern: Differential geometry of fiber bundles (pdf, djvu)
- Whitehead: Homotopy groups of spheres (pdf, djvu)
- Eckmann: Complex-analytic manifolds (pdf, djvu)

The full list of ICM 1950 articles is here: Volume I, Volume II.