Big list of repositories of mathematical preprints and postprints I'm looking for a extensive list of online repositories of mathematical preprints and postprints. I'm interested in every type of repository, including small informal and semi-formal collections, like for example the webpage of Ranicki, collecting various notes on surgery theory, and the Clay Foundation's online collections, like the one with the collected works of James Arthur. Surely there are many other similar sites out there.
Here is a related question, which asks specifically about repositories for theses.
 A: *

*Linear algebraic groups and related structures, papers related to linear groups from 1998 to present.

*K-theory preprint archives for K-theory related papers between 1994 and 2012 (with a recent addition on the end of the Journal of K-theory)

*Hopf topology archive, algebraic topology related papers until 2010
A: The french repository HAL might be of interest too:
HAL
A: *

*Max Planck Institute for Mathematics preprint series

*Max Planck Institute for Mathematics in the Sciences preprint series
A: In addition to arXiv as already mentioned, see too this directory of mathematics preprint servers:

It makes available the current homepage URLs and email contacts of
  mathematical preprint and e-print servers throughout the world. It is
  a tool to help find these servers in order to browse the articles
  posted on them or to post an article to the server itself. The servers
  are divided into three categories: umbrella servers which cover all
  areas of mathematics such as the Mathematics ArXiv and the
  MPRESS/MathNet.preprints server, special subject servers and servers
  administered by mathematics departments and institutes. There is an
  additional link to retired preprint services.

Insofar as mathematics and cryptography intersect, I would suggest the IACR cryptography eprint service.
Regarding viXra, we discussed it over on Math SE awhile ago. It is distinct from the snarXiv.
A: There is viXra.org.
I don't really know what the incentive is to post things there rather than arXiv, and the quality is usually lower, but I have found a couple papers which I thought were interesting.
A: There is a fairly large server for preprints in mathematical physics at U of Texas
Mathematical Physics Preprint Archive mp_arc
A: There are links here to many papers by the late Alf van der Poorten. 
The collected papers of Paul Erdos are available here. 
A: *

*The Kurt Mahler archive: http://carma.newcastle.edu.au/mahler/

*A repository of e-prints about optimization and related topics: http://www.optimization-online.org/
A: *

*Repository of the University of Jyväskylä (not limited to mathematics), JYX.

*Department of Mathematics and Statistics (University of Jyväskylä): reports and preprints.

A: *

*The preprint server of the research group in Calculus of Variations and Geometric Measure Theory at Pisa: http://cvgmt.sns.it/
(A similar project for Complex Geometry and Differential Geometry is at http://gecogedi.dimai.unifi.it/, but preprint functionality is not (yet) available.)
A: *

*http://dl.acm.org/  "Full text of every article ever published by ACM [Association for Computing Machinery]".

*http://ieeexplore.ieee.org/  Other papers in (theoretical) computer science.
Unfortunately many papers on both sites are paywalled. But the full text of many of these can be found via Google.
A: For completeness, I guess it's worth mentioning the obvious repository, the arxiv, specifically the mathematics section.
A: Groups, Representations and Cohomology Preprint Archive maintained by Dave Benson.
A: Real Algebraic and Analytic Geometry preprint server,
http://www.maths.manchester.ac.uk/raag/
A: No one has mentioned the AMS repository at BROWN
AND THE MAA at UTexas?
