Google Scholar is a Google search engine for articles.
CERN Preprint Server
"Over 650,000 bibliographic records, including 320,000 full text documents . . . in particle physics and related areas. Covers preprints, articles, books, journals, photographs, and much more."
SPIRES High Energy Physics
"Search more than 500,000 high energy physics related articles, including journal papers, preprints, e-prints, technical reports, conference papers and theses." SPIRES HEP is a joint project of Stanford Linear Accelerator Center (SLAC), DESY & FNAL and the worldwide HEP community.
NASA Astrophysics Data System
NASA-funded Astrophysics Data System (ADS) maintains four bibliographic databases containing more than 3.9 million records: Astronomy and Astrophysics, Instrumentation, Physics and Geophysics, and preprints in Astronomy." Try the new ADS Bumblebee Interface for an enhanced searching experience.