It seems that neither option presented below are taking new submissions. I keep the answer here for historical interest.
One option is:
- Philica which occupies a bit of a strange place. It is a free, open-access journal that publishes immediately and in any discipline. The website comes with a non-traditional review system. It is in between what one may call a pre-print server and what one would call an electronic open access journal.
Nature Precedings used to take submissions, but no longer does:
- Nature Precedings: a pre-print repository run by Nature Publishing Group that focuses on chemistry, biological sciences, and earth sciences. Edit: As bobthejoe noted below in the comments, Nature Precedings is no longer taking new submissions; though it will for the foreseeable future remain a repository for the pre-prints already submitted.