These are some good references for questions that involve Biology, Ecology, Chemistry, Physics and the people that work in these fields.

Atmosphere/Meteorology

http://www.noaa.gov/

The National Oceanic and Atmospheric Administration website has a ton of useful information on everything from weather to climate to fisheries to costal wetlands.  There is even a section of the site that allows you to see real-time data from their satellites.  NOAA also links to the National Weather Service.

Biology

http://www.eol.org 

This is the Encyclopedia of Life, a project that devotes pages to every identified species on the planet.  I don't mean to gush, but this stuff is really cool, and Chicago's own Field Museum is one of a short list of Partners for the EOL project.

http://plants.usda.gov

This site is published by the United States Department of Agriculture and includes information on all things plant. 

Ecology

http://www.earthtrends.wri.org

This page is maintained by the World Resources Institute.  It has many maps and statistics about ecosystems as well as information on climate and conservation.

Physical Sciences

http://scienceworld.wolfram.com

This page is a very good resource for Physics, Chemistry, Astronomy and Math and also includes a number of excellent biographies of prominent scientists.

Astronomy/Space

http://www.nasa.gov

They managed to put people on the moon, and spectacles on the Hubble Telescope, who could possibly know more about Astronomy and Space?

Miscellaneous

http://www.madehow.com

Ever wonder how it was made?  These folks probably know.

http://www.timeanddate.com

Need an alibi?  In addition to time zone information and time conversion calculators, this also has a great prepetual calander function. 

http://www.almanac.com

The Farmers' Almanac.  Some would argue that this should be the first reference on this page.