MGNet Preprints - Multigrid and other methods for PDE simulation. (100+ articles)

K-theory Preprint Archive - K-theory (generalized cohomology theories). (~400 articles)

Mathematical Physics Archive - Also called mp_arc; at UT Austin. (2000- articles)

European Singularities Network - Singularities in geometry and complex analysis. Searchable index (100+ articles).

Algebraic Number Theory Archive - Number theory/arithmetic geometry. (~200 articles)

Topology Atlas Preprints - Mostly general topology. (400+ articles)

Conservation Laws Preprint Server - Analysis/simulation of conservative PDEs. (100+ articles)

Banach Space Bulletin Board - Banach spaces/functional analysis. (~400 articles, arXiv overlay)

Cryptology ePrint Archive - of the IACR

Groups, Representations, and Cohomology Preprint Archive - Algebraic topology/finite group theory. (100s of articles.)

Markov Chain Monte Carlo Preprint Service - A list of all registered papers on MCMC methodology currently submitted for publication. Based at the Statistical Laboratory of the University of Cambridge.

Field Arithmetic Archive - Galois theory of fields/group theory. (50- articles)

Shelah's Archive - Papers by Saharon Shelah in logic and related. (100s of articles, arXiv mirrored)

Linear Algebraic Groups and Related Structures - and related structures (~50 items)

Online Theses in Number Theory - ~35 items.

Probability Abstract Service - Probability Abstract Letters are sent about once every two months. The subscription is free.

Evolve-L archive - Evolution equations and semigroups. (100- articles)

Optimization Online - A repository of eprints about optimisation and related topics.

Jordan Theory Preprint Archives - Jordan algebras and related topics (~100 articles).

MG-Net Preprints - Multigrid methods. Compressed PostScript, indexed by author.