ArchaeoGlobe: Global Assessment of Archaeological Knowledge on Land Use
http://globe.umbc.edu/archaeoglobe/ The ArchaeoGlobe Project is a “massively collaborative effort” to assess archaeological knowledge on human land use across the globe over the past 10,000 years.
CLIMAS: Climate Assessment for the Southwest
http://www.climas.arizona.edu/
CLIMAS is a collaborative effort to pull together weather and climate data and monitoring networks including the Cooperative Observer Program (COOP); Historical Climate Network (HCN); Arizona Meteorological Network (AZMET); PRISM; and Remote Automated Weather Stations (RAWS)
Climate Reanalyzer
http://cci-reanalyzer.org/
Climate Reanalyzer is a platform developed by the Climate Change Institute (University of Maine) that draws on climate reanalyses, models and historical station data to create different plot maps, correlations, and timeseries.
GBIF: Global Biodiversity Information Facility
http://www.gbif.org/
The Global Biodiversity Information Facility (GBIF) is an international open data infrastructure, funded by governments, that provides free and open access to biodiversity data.
GHCN: Global Historical Climatology Network
http://www.ncdc.noaa.gov/data-access/land-based-station-data/land-based-datasets/global-historical-climatology-network-ghcn
The GHCN is an integrated database of climate summaries from land-surface stations across the globe that have been subjected to a common suite of quality assurance reviews. The data are obtained from more than 20 sources. Some data are more than 175 years old while others are less than an hour old. ftp://ftp.ncdc.noaa.gov/pub/data/ghcn/
GLOBE: Global Collaboration Engine
http://globe.umbc.edu/ GLOBE (Global Collaboration Engine) is an online collaborative environment that enables land change researchers to share, compare and integrate local and regional studies with global data to assess the global relevance of their work.
GNIP data: Global Network of Isotopes in Precipitation
http://www-naweb.iaea.org/napc/ih/IHS_resources_gnip.html. This is a service of the Water Resources Programme of the International Atomic Energy Agency whose focus is to serve as an observation network of stable hydrogen and oxygen isotope data for hydrologic studies. This isotope database has been used in: (i) verifying and improving atmospheric circulation models, (ii) studying regional, global and temporal climate, (iii) studying the interactions of water between atmosphere and biosphere, and (iv) providing baseline information for the authentication of commodities, such as food, plants, and for tracking migratory species (birds, fish, butterflies etc.), and for forensic purposes.
ITRDB: International Tree-Ring Data Bank
http://www.ncdc.noaa.gov/data-access/paleoclimatology-data/datasets/tree-ring
The largest international archive of tree-ring data. Includes searchable datasets, information about raw ring width, wood density, isotope measurements, site growth index chronologies, some reconstructed climate parameters, and interactive maps.
ftp://ftp.ncdc.noaa.gov/pub/data/paleo/treering/
LANDFIRE: Landscape Fire and Resource Management Planning Tools http://www.landfire.gov/
LANDFIRE provides geospatial data and databases that describe vegetation, wildland fuel, and fire regimes across the United States and insular areas
NAEBD: North American Ethnobotany Database
http://herb.umd.umich.edu/
A database of plants used by native Peoples of North America as drugs, foods, dyes, fibers, and more. Many species records are linked to the USDA plant database.
National Geographic Web Site for Printing USGS Maps
http://www.natgeomaps.com/trail-maps/pdf-quads
Allows you to download an print any USGS quad in the US.
NCDC: NOAA–National Centers for Environmental Information
http://www.ncdc.noaa.gov/data-access/paleoclimatology-data/datasets
Provides 18 separate datasets and associated information that address paleoenvironmental reconstruction. Among these are the International Tree-Ring Data Bank (ITRDB) and datasets pertaining to macrofossils, pollen, ice cores, and fire histories
NED: National Elevation Dataset
http://ned.usgs.gov, ftp://rockyftp.cr.usgs.gov/vdelivery/Datasets/Staged/NED/
NED is a raster product provided by the U.S. Geological Survey that provides national elevation data layers available in several horizonal resolutions
NEON: National Ecological Observatory Network
http://www.neoninc.org
A continental-scale observatory (106 sites across US) that measures causes and effects of climate change and land-use change in various U.S. ecosystems
Neotoma: Neotoma Paleoecology Database
http://www.neotomadb.org, http://api.neotomadb.org/
The Neotoma database is an international collaboration among 19 institutions that provides cyberinfrastructure permitting scientists to input, discover, and analyze environmental data (middens, pollen, insects, ostracods, etc.) associated with temporal contexts from the Pliocene to the Quaternary
NHD: National Hydrography Dataset
ftp://nhdftp.usgs.gov/DataSets/Staged/
Database that portrays surface water on the National Map, supplied by the US Geological Survey
NRCS SSURGO: National Resources Conservation Service-Soil Survey Geographic Database
http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/survey/geo/?cid=nrcs142p2_053627
http://websoilsurvey.sc.egov.usd.gov/App/HomePage.htm
Database of soil information and maps collected by the National Cooperative Soil Survey
PaleoFire
http://www.paleofire.org
A global paleofire dataset for research and archiving sedimentary records of fire. The GCD is managed by the Global Paleofire Working Group
PaleoView
https://github.com/GlobalEcologyLab/PaleoView/releases
A freeware software tool for generating and viewing paleoclimate data at temporal and spatial resolutions suitable for detecting biotic responses to major climate shifts over the last 21,000 years.
PDSI: Palmer Drought Severity Index
http://www.drought.gov/drought/content/products-current-drought-and-monitoring-drought-indicators/palmer-drought-severity-index
A tool to comprehensively asses moisture status using temperature and precipitation data, as well as soil moisture, to assess water supply and demand.
PRISM: Parameter-elevation Relationships on Independent Slopes Model
http://www.prism.oregonstate.edu/
PRISM takes climate datasets from a wide range of monitoring networks and provides spatial climate datasets at multiple spatial and temporal resolutions.
SEINET: Southwest Environment Information Network
http://swbiodiversity.org/seinet/
A database of data resources from collections and observations from museums and agencies for environmental research within Arizona
TreeFlow
http://treeflow.info/
Long term streamflow reconstructions for the US organized by major river basins.
WorldClim: WorldClim-Global Climate Data
http://www.worldclim.org
WorldClim provides global climate layers (climate grids) with a spatial resolution of about 1 square kilometer.
WRCC: NOAA–Western Regional Climate Center
http://www.wrcc.dri.edu
WWRCC is a regional repository of historical and current climate data and information for the Western US. Available data include precipitation, temperature, freeze probabilities, growing degree days, heating and cooling degree days—normals, means, and extremes at various temporal scales for thousands of stations; Desert Research Institute, Reno.
Suggest a resource to add in comments, below.