I usually (but not always) include links to data on which my posts are based. It occured to me that it would be useful to collect data links in a single location. Therefore this page is a holding area for links to climate data.
It’s just starting, so at the moment it’s not nearly complete, and some of the links below are not yet active! Give it time…
Surface temperatures:
NASA GISS (NASA Goddard Institute for Space Studies)
NCDC (National Climate Data Center)
HadCRU (Hadley Centre/Climate Research Unit)
ECA (European Climate Assessment & Dataset Network)
GHCN version 1, especially for data prior to 1880.
GSOD, Global Surface Summary of the Day (from NCDC)
Satellite-based atmospheric temperature estimates:
RSS (Remote Sensing Systems)
UAH (University of Alabama at Huntsville)
UW-RSS (University of Washington/RSS)
UW-UAH (University of Washington/UAH)
UMD (University of Maryland)
Sea Ice
HadISST (Hadley Centre/Climate Research Unit)
Sea Ice Extent and Area Data (National Snow and Ice Data Center)
Daily data for sea ice extent from the Japan Aerospace Exploration Agency
Daily Data from GSFC.
Daily N.Hem Sea Ice Area from Cyrosphere Today
Daily S.Hem Sea Ice Area from Cyrosphere Today
Daily Global Sea Ice Area from Cyrosphere Today
Sea Level
Satellite-derived Sea Level from the sea level site of the University of Colorado.
Snow Cover
Rutgers University global snow lab
Total Solar Irradiance (TSI)
PMOD composite of satellite data
ACRIM composite of satellite data
IRMB composite of satellite data
Lean (2004) (reconstruction by proxy)
Multiple reconstructions of TSI.
Sunspot Numbers
Greenhouse Gases
Mauna Loa Atmospheric Observatory CO2
WDC for Greenhouse Gases
Daily and Hourly CO2 data
Climate Sensitivity
Data Archiving Pages:
ESRL/CDC (Earth System Research Laboratory/Climate Data Center)
ESRL/CDC timeseries (time series of climate variables)
CDIAC (Carbon Dioxide Information Analysis Center)
Monitoring and Data Index from NOAA/NWS Climate Prediction Center
Australia’s Bureau of Meteorology Climate Change page
Visualizations etc.
Royal Netherlands Meteorological Institute
Weather Data Navigator from NOAA
Computer Model Results
Open Mind 
I would include:
Carbon Dioxide Information Analysis Center (Oak Ridge National Laboratory)
http://cdiac.ornl.gov
This is where I have found the ice core data for the past millenia, e.g.,
Law Dome Ice Cores
Historical CO2 Records from the Law Dome DE08, DE08-2, and DSS Ice Cores
http://cdiac.ornl.gov/trends/co2/lawdome.html
… but they go beyond just the estimation of CO2 levels, looking into the carbon cycle and sequestration levels, emission levels, methane, etc..
http://arctic.atmos.uiuc.edu/cryosphere/
Can’t live without.
Tamino, your link to “Mauna Loa Atmospheric Observatory CO2” is broke.
http://scrippsco2.ucsd.edu/data/data.html
for co2 data
thanks for this page
Tamino, the link to Mauna Loa CO2 is still wrong (it points right back at this same page).
jl provides the URL for CO2 data from Mauna Loa at Scripps. There’s also this site for various gas measurements. For the limited number of stations included, it seems to be more up to date than the WDC link you provide. Maybe include both of them?
ftp://ftp.cmdl.noaa.gov/ccg/