Real Climate – Climate Science from Climate Scientists

http://www.realclimate.org/

This website provides a one stop link for resources that people can use to get up to speed on the issue of climate change:

For complete beginners:

NCAR: Weather and climate basics
Oxford University: The basics of climate prediction
Pew Center: Global Warming basics
Wikipedia: Global Warming
NASA: Global Warming update
National Academy of Science: Understanding and Responding to Climate Change
Encyclopedia of Earth: Climate Change Collection
Global Warming FAQ (Tom Rees)
Global Warming: Man or Myth? (Scott Mandia, SUNY Suffolk)

There is a new booklet on Climate Literacy from multiple agencies (NOAA, NSF, AAAS) available here (pdf).

Those with some knowledge:

The IPCC AR4 Frequently Asked Questions (here) is an excellent start.