Preprint services other than arXiv (for other fields)
The OSF provides a general open preprint infrastructure that is connected to a range of preprint services. Importantly, it is not owned by a commercial publisher.  It supports a number of discipline-specific preprint services many of which use the ArXiv name under licence.

The list of preprint services is growing over time. 
For further information go to: https://osf.io/preprints 

In general, no matter what the discipline you can post to:


OSF preprints https://osf.io/preprints/


However, the OSF supports the following discipline-specific preprint services. Presumably, if your preprint aligns with any of these disciplines, then you would be better off posting to them.


Psychological Sciences.  PsyArXiv https://osf.io/preprints/psyarxiv/
Social Sciences. SocARXIV https://osf.io/preprints/socarxiv
Engineering. engrXiV https://osf.io/preprints/engrxiv
Agriculture and Allied Sciences. AgriXiv https://osf.io/preprints/agrixiv


More discipline-specific preprint services using the OSF framework are being added on a regular basis: https://cos.io/blog/public-goods-infrastructure-preprints-and-innovation-scholarly-communication/

Useful features of OSF-based preprint services


Strategy for long term archiving
Integration with Google Scholar
Integration with OSF projects which allows you to link other materials such as data, code, and materials
OSF is a not for profit entity run by academic researchers (contrast this with SSRN, Figshare, ResearchGate; i.e., no ads and goals aligned with academic community)
The functionality of OSF preprints is improving on a regular basis. See features road map
You can choose a licence
You can link to the doi of the subsequently published manuscript.