SPARC Data Center (Sun, O3 and volc. data)
SPARC Data Center home page
Neutron monitor data, University of Chicago
ACRIM composite irradiance data (1978-2003)
WDC C1, World data centre for geomagnetism at DMI, Copenhagen
NGDC home page with solar data (sunspot numbers, geomagnetic indices, etc.)
Solar Cycle Length values according to the 121 and 12221 scheme.

Hadley Centre radiosonde data sets
CRU home page for various mean instrumental temperature series
NCEP Reanalysis data
ERA Reanalysis data
University of Arizona Dendrochronology collection of climate links