The [Keepers Registry](http://thekeepers.org/) currently has 8 participating archiving agencies:

* [British Library](http://www.bl.uk/)
* [CLOCKSS Archive](http://www.clockss.org/)
* [Global LOCKSS Network](http://www.lockss.org/)
* [HathiTrust](http://www.hathitrust.org/)
* [Portico](http://www.portico.org/)
* [Archaeology Data Service](http://archaeologydataservice.ac.uk/)
* [National Science Library, Chinese Academy of Sciences](http://english.las.cas.cn/)
* [e-Depot](http://www.kb.nl/en/expertise/e-depot-and-digital-preservation)

A quick look at [their descriptions](http://thekeepers.org/thekeepers/keepers.asp?action=agencies) suggests that some may be interested in archiving MathOverflow.

The [Center for Research Libraries](http://www.crl.edu/archiving-preservation/) appears to be the main international entity for assessing and certifying the trustworthiness and reliability of major archiving agencies. Some reports can be found [here](http://www.crl.edu/archiving-preservation/digital-archives/certification-and-assessment-digital-repositories).