What are the best open-access journals in physics? I would like to support open-access journals by choosing to publish in journals which allow readers free online access.    Ideally I would also like to retain copyright instead of signing it over to the journal.
What are some of the better open-access journals in physics?  I am particularly interested in journals focused on optics, but let's keep the question general.
 A: The New Journal of Physics has done a pretty good job attracting serious papers, and there are a number of quite good cold atoms/condensed matter/quantum optics papers.
APS is new to the open access game, but I've no doubt that PRX (which I think, as a community, we should all refer to as Phys. Rev. eXtreme...) will be a good resource when it debuts.
edit: fixed so journal titles are links
A: Optics express is excellent. Fast turnaround, high impact factor, and decent peer review. 
A: arXiv
Though it is not a journal in the classical meaning (it is a repository of preprints), to some extent it serves as such. So when you care for the open-access, advisable it to upload your paper to arXiv and then send to a peer-review journal (open or not).
Sending a paper to arXiv give also other benefits - it is (almost) immediately accessible to the scientific community. So it both boosts the research process and prevents form being scooped.
(And out of my personal philosophy - publicly funded research should be released openly, not locked in private hands.)
A: With regard to open access publication of conference proceedings, I draw your attention to Proceedings of Science http://pos.sissa.it/POSwhat.html . From their webpage:
"The aim of PoS is to offer a versatile, fast, inexpensive and open tool devised and run by the scientific community for the scientific community. The service will hence be open to all readers free of charge and the fees for publication will be kept low on a not-for-profit basis.
Given the Open Access nature of this proceedings collection, PoS has chosen to apply the Creative Commons License (Attribution-NonCommercial-ShareAlike). Every author submitting his/her contribution to PoS retains the copyright, and upon submission (s)he applies this license to his/her work."
(emphasis mine).
