How do I find climate data for my research?

Once every three weeks a colleague (usually in the social sciences) emails me to ask about whether a climate data set of type X exists or what the pros/cons of various datasets are. I'm always happy to help, but it's useful to  know that there are multiple websites that maintain collections of various kinds of climate/global environment data, and several have pages that specifically compare the array of data sets out there.  If you're looking for data, a few clicks through these sites will give you a good sense of what's available.

• NCAR's Climate Data Guide (new and very nice)
• Columbia's IRI/LDEO Climate Data Library (interfaces with OPeNDAP, making download easy)
• NOAA's Earth System Research Laboratory Gridded Data Collection
• Reanalysis.org (discussed briefly here)
• IPCC Data Distribution Center (both historical and future projections are here)
• RealClimate Data Sources Page

All of these sites are listed in our meta-resources page.