I use plenty of free resources to make the maps that appear on this website and on my blog DAMWeather. All of my maps are created using QGIS, a powerful and free GIS program that's comparable to ArcGIS. Many of these links are easily accessible in a more compact format on my reference desk pages.
Base Layers
Natural Earth – The ultimate free resource for basic GIS mapping data.
Census TIGER – Basic but detailed shapefiles for boundaries and more, via the U.S. Census Bureau.
Earth Explorer – An in-depth resource from the USGS, especially for satellite imagery and elevation models.
NASA Blue Marble – GIS-friendly rasters of Blue Marble satellite imagery.
Weather Portals
NDFD Degrib – An essential decoding program that downloads data from the NWS and decodes it for you into GIS-ready formats like shapefiles and NetCDF.
NWS Web Services – A comprehensive NWS portal for all of the agency's freely available GIS services.
National Hurricane Center – The NHC's portal for current and past storms. Data released in shapefiles and KML files.
Weather Prediction Center – The WPC's portal for precipitation outlooks, surface analyses, and more.
Iowa Environmental Mesonet (IEM) – An exceptional and in-depth resource for current and past weather data.
Current Weather
SPC Convective Outlooks: Day 1 | Day 2 | Day 3 | Day 4 | Day 5 | Day 6 | Day 7 | Day 8
SPC Fire Outlooks: Day 1 | Day 2 | Day 3 | Day 4 | Day 5 | Day 6 | Day 7 | Day 8
IEM: Current Watches | Storm Reports (past 24 hours)
U.S. Satellite Imagery: GOES-West | GOES-East
Full Disk Satellite Imagery: GOES-West | GOES-East
Sea Surface Temperatures – A collection of sea surface temperature data from NOAA Coral Reef Watch.
Past Weather
IBTrACS – The official collection of all documented tropical cyclones around the world.
Storm Prediction Center – The SPC provides data on tornadoes and reports of damaging winds and large hail since 1950. Data usually lags about two years behind present.
NWS Damage Assessment Toolkit – Official tornado survey results and imagery from the National Weather Service. Data is downloadable for GIS software.
NOAA GridSat – An archive of GOES-11 and GOES-12 data from 1994-2017.
IEM Archived Storm Reports – GIS-ready data on all local storm reports, updated every couple of minutes.
IEM Archived NWS Warnings – GIS-ready archives for just about all watches, warnings, and advisories issued by the National Weather Service since 1986.
IEM Archived Radar Imagery – Decades of archived radar imagery, including national composites and higher-resolution local radar sites.
Canadian Tornadoes (1980-2009) – Environment Canada's archive of confirmed tornadoes across Canada between 1980 and 2009.
Northern Tornadoes Project – A modern and in-depth archive of confirmed tornadoes across Canada since 2017, by Western University.