http://www.esri.com/
The main provider of GIS software provides an extensive array of guidelines and tools online.
CRSEU
http://crs.bkg.bund.de/crs-eu/
The Information and Service System for European Coordinate Reference Systems (CRS) provides guidance in solving problems with e.g. projections and transformation.
EIONET
http://www.eionet.eu.int/gis
The European Environment Information and Observation Network (EIONET) contains links to guidelines, templates and services for use when handling spatial data, maps and geographical information systems.
Data and Maps
EEA Dataservice
http://dataservice.eea.eu.int/dataservice/
The dataservice of the European Environmental Agency provides almost all data sets and applications which have been used in EEA's periodical environmental reports.
ESDI (Earth Science Data Interface)
http://glcfapp.umiacs.umd.edu:8080/esdi/index.jsp
The Earth Science Data Interface is the data-providing portal of The Global Land Cover Facility. GLFC is a research program under NASA, hosted by University of Maryland. Here you can find some of the most comprehensive global datasets, including the MODIS vegetation cover maps, GIS layers of IUCN protected areas, as well as the AVHRR global land cover and primary production maps.
GEO
http://geodata.grid.unep.ch/
The GEO data portal of The United Nations Environment Programme. It holds many geospatial maps, covering themes like Freshwater, Population, Forests, Emissions, Climate, Disasters, Health and GDP. These are however not available in GIS-format.
Geography Network
http://www.geographynetwork.com/
The Geography Network is a global network of geographic information users and providers, managed and maintained by GIS manufacturer ESRI. Datasets include global forest cover maps and Geographic Names Information data, and there are dynamic links to many other facilities.
ICES
http://www.ices.dk/datacentre/data_intro.asp
ICES (I`nterbational Council for Exploration of the Sea) is the organisation that coordinates and promotes marine research in the North Atlantic. Their portal contains many oceanographic and realted datasets.
USGS
http://gisdata.usgs.net/
United States Geological Survey (USGS) is the extensive source for all kinds of GIS maps covering the USA.
LPDAAC
http://edcdaac.usgs.gov/dataproducts.asp
The Land Processes Distributed Active Archive Center (LPDAAC) disseminates global land processes data, collected by various satellite and aerial sensors. Datasets available here include Terra ASTER, MODIS vegetation cover, Landsat Pathfinder, the Global 30 Arc-Second Elevation Data Set and others.