ArXiv (http://arxiv.org/) has been available for some years for storing preprints of articles on Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance and Statistics.
A newer development is BioRxiv (http://biorxiv.org/) for preprints related to Biology. This archive has a section dedicated for Plant Biology. I found some interesting preprints in it.
It will hopefully become an important resource for our research community.