Google Scholar searches a large variety of indexed material (ArXiv, Physical Review journals, etc.) with the familiar Google interface and effectiveness.
ArXiv : This is a great repository for preprints of articles in major journals as well as postprints, articles, conference proceedings and more. One feature is that it allows you to narrow your search to very specific sub-disclines in physics.
List of Open Access Physics Journals: This is a link to the Directory of Open Access Journals, specifically their physics listing.
And just to spread the word, if you are worried about Open Access or open access mandates infringing on academic freedom, please read: Open Access: Six Myths to Put to Rest