ACS has launched ChemRxiv.
Here's an excerpt from the announcement:
ChemRxiv, a new chemistry preprint server for the global chemistry community, is now available in a fully functioning Beta version for use and feedback by researchers. The Beta launch has been undertaken with initial strategic input from the American Chemical Society (ACS), Royal Society of Chemistry, German Chemical Society and other not-for profit organizations, as well as other scientific publishers and preprint services. The free-of-charge service, originally announced late last year, is managed on behalf of the chemical science community by ACS and is powered by Figshare, an online digital repository for academic research.