Science, generally; Physical Sciences; Technology

Science, generally

Armchair Scientist
Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP), publishers of the Skeptical Inquirer
Echo Research Center - Directory of sites on the history of science, technology, and industry (George Mason University)
Exploratorium: Ten Cool Sites - Monthly list of recommended science sites, with archive of past picks
Intute: Science, Engineering, and Technology - Tens of thousands of links
NSDL Scout Reports - Three reports were published: life sciences; physical sciences; and math, engineering, and technology. Ceased publishing in 2005
Powers of Ten - Learn about all the sciences through multiples of ten (meters, seconds, etc.)
psci-com - Guide to Internet sources that inform the public about science and technology. Includes exemplary science Web sites, science museums, science in society, etc. (a Wellcome gateway)
Science Reference Services (Library of Congress)

Eric Weisstein's World of Science - Mathematics, Physics, Chemistry, Astronomy, Scientific Biography
Why Files? Science Behind the News
See also Open Access Journals and Skepticism

Questions and Answers

Ask the Experts at Scientific American
Last Word - Why is the sky blue? Why is yawning contagious? Your science questions answered (New Scientist)
Mad Sci Network - The mad scientists answer your questions
Everyday Mysteries (Library of Congress)

Some of these sources have RSS feeds, which are marked with (What's this?)

EurekAlert - News in all branches of science (American Association for the Advancement of Science)
Knight Science Journalism Tracker - Top science stories of the day (MIT)
Science Daily - News
Scientific American
SciTech Daily Review - Science's answer to Arts and Letters Daily

Scirus - Search engine for science Web sites and scientific research articles (from ScienceDirect, Medline, BioMed Central, and other sources). There is a fee to get the full text of some articles.
Canada Institute for Scientific and Technical Information (CISTI) - CISTI is a combination library and publisher. You can search its holdings for scientific and technical literature and have documents delivered to you (for a fee)
CiteSeer - An experiment in automated indexing
Institute for Scientific and Technical Information - Article and monograph index and document delivery service from France's Institut de l'Information Scientifique et Technique (this link takes you to the English-language interface) [Inist.fr]
National Science Digital Library (NSDL) - Search across many quality science sites
SAIL E-prints - A database of multiple e-print servers in chemistry, physics, engineering, computer science, earth sciences, agriculture, and related fields
SciCentral - Gateway to scientific research news sources
Science Direct - Index to Elsevier science journals. There is a charge to get the full text of articles.
Scientific Societies (U.S. Dept. of Energy, OSTI)

Science.gov - Portal to U.S. government research; search multiple federal databases at once. Alerts available.
Defense Technical Information Center (DTIC) - Research done for the Dept. of Defense. Unclassified material is available through the Public STINet (Scientific and Technical Information Network) service
Federal R&D project Summaries
National Academies Press - Publishes reports by the National Research Council, National Academy of Sciences, National Academy of Engineering, and Institute of Medicine — many of them available in full text.
National Technical Information Service (NTIS) - Technical publications from government agencies, such as the U.S. Environmental Protection Agency. The Web database goes back only to 1990, so e-mail or call for publications older than that.

Although these services come from the Dept. of Energy, all of them cover a wider range of scientific literature than just energy.

E-print Network - Portal for e-print sites
Information Bridge - Database of DOE research in full text (mostly from 1995 on)
See also Energy Index, below

arXiv - Preprints in physics, math, nonlinear science, computer science, and quantitative biology (Cornell and Los Alamos National Laboratory)
CERN Document Server
HEP Search: High-Energy Physics Literature Database (Stanford Linear Accelerator, SPIRES)
NASA Astrophysics Data System - Indexes literature in astronomy, astrophysics, geophysics, physics, and instrumentation

ChemWeb - Search preprints and abstracts of journal articles. Pay-per-view to get full text of journal articles. Free registration required.
Chmoogle - Search for information on chemicals
PubChem - "Information on the biological activities of small molecules" (National Library of Medicine)

Energy PolySearch Engine - Péter Jacsó's page for searching multiple energy sites

Energy Citations - Index going back to 1948 (Office of Scientific and Technical Information, U.S. Dept. of Energy)

Figure out conversions between one unit of measurement and another.

Conversion and Calculation Center - Measurements, currency, time zones, calculators (Convert-It.com)
Measurements Converter (Convert-Me.com)
Online Conversion.com
World Wide Metric Conversion Calculator
WWW Unit Converter
See also Calculators and Time and Temperature