This list is maintained by Penn State. Some interesting selections:

• The AMS Preprint Server
• alg-geom Algebraic Geometry at Duke University.
• MSRI E-print Archive at the Mathematical Sciences Research Institute, Berkeley Includes e-print libraries on:
  • Differential geometry and global analysis (dg-ga)
  • Automorphic forms
  • Complex dynamics and hyperbolic geometry
• The xxx Physics E-print Archive. The definitive physics e-print archive. Has a huge collection of papers, and some of the most advanced e-print software. Most of the other E-print servers use software developed at xxx.
• Banach Spaces and Functional Analysis ftp at Oklahoma State
• mp_arc Mathematical Physics Preprint Archive at University of Texas
• Dynamical Systems at SUNY Stony Brook

The Penn State lists above are probably as good as any:

• The Mathematics Section of the WWW Virtual Library includes:
  • List of Journals
  • List of Preprints
• A list of Online Publications, Techreports, Preprints at Simon Fraser University.