Cite Seer. Digital library for computer and information science literature.
Energy Citations Database. Access to citations and full-text articles from 1943-present in various science fields.
NASA. National Aeronautics and Space Administration.
Science Accelerator. Documents from a wide variety of publication types, moderated by the Department of Energy.
Science Research: A free, publicly available deep web search
engine that uses advanced "federated search technology" to return high
quality results by submitting your search query - in real-time - to
other well respected search engines then collating, ranking and
dropping duplicates of the results.
Scitopia. Digital library of 21 leading science and technology societies.