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.

• 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.

• 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.

• EPA Climate Change. Covers a wide range of climate change topics. Most are aimed at the general public.
• Wikipedia: Climate Change. A good overview.
• RealClimate 'Climate Science from Climate Scientists'