Figshare is a rather new service (compared to ArXiv) that is just starting to gain momentum. I haven't used it personally, but they have partnered up with some other Open Access players, most notably and recently with PLoS (Figshare will host supplemental data for all PLoS journals). As far as I know, there is no restriction regarding the fields the submissions must be in.
It is a repository, the service is free (unlimited public posts, 1GB private posts). No review or moderation; the submissions will be posted immediately under CC-BY. Each submission is given a DOI. I cannot find info about how many submissions they have received and posted.
NOTE: Just checked with Figshare through twitter, and after one year they have 200,000 files shared by users (could be papers, figures, charts, data, etc)
Edit: just realized I should also mention the following:
PeerJ was just launched very recently (like in the last month or two I think). It's a journal but has its own pre-print system. You can submit unlimited number of public pre-prints in their preprint server PeerJ Preprints, which has its own ISSN number. You will be able to submit your pre-prints subsequently to their peer-reviewed journal. The journal is gold OA and charges what I believe is a one-time membership fee. The journal has limited scope (does not publish in the Physical Sciences, the Mathematical Sciences, the Social Sciences, or the Humanities ), not sure what the pre-print service will look like yet and it is worth checking out later on.
