There are a lot of mediocre climate sites out there, but I've found some authoritative sites that I can recommend. Please let me know of any other good links that I could add.

Overviews      Specialized Topics      Global Warming     

• Climate entry in Wikipedia. Very good intro, with many subtopics.
• Köppen climate classification entry in Wikipedia. How climate type affects vegetation, etc.
• Wind entry in Wikipedia.
• Rain entry in Wikipedia.
• Climatology and Meteorology entries from Wikipedia.
• Meteorology from the University of Illinois. Online Instructional Modules (lessons, in other words) aimed at students. Covers weather rather than climate, but quite useful.
• Global Climate Animations from the University of Oregon. Temperature, Water Balance, Winds, many other in nice animated displays.

• Acid rain entry in Wikipedia.
• NOAA Arctic Theme Page Focus on Arctic Climate and related topics.
• NOAA Climate Portal. Many rich, authoritative sites. NOAA has more data than anyone, and they make it all available, sometimes free, sometimes not.
• NASA Climate Links. Mostly advanced topics. Naturally this heavily involves satellite sensing of one kind or another.
• Snow and Ice Data from the National Snow and Ice Data Center, part of the University of Colorado Cooperative Institute for Research in Environmental Sciences, affiliated with NOAA. Glaciers, paleoclimate, sea ice, much more. Mostly quite technical.

• Understanding Climate Change from the National Center for Atmospheric Research (NCAR). Very Good.
• EPA Climate Change Indicators. Covers a wide range of climate change topics. Most are aimed at the general public.
• Wikipedia: Global Warming. A good overview.
• RealClimate 'Climate Science from Climate Scientists'