http://shapewiki.com is a tagged, collaboratively populated collection of mapping shapes. Export in JS/JSON/XML.

http://finder.geocommons.com/ is a collection of user uploaded data. Export in Spreadsheets/KML.

http://www.dyngeometry.com/  provides a JSON API for retrieving border data for world countries/regions. Preview live on the site.

World Datasets:

 
Digital Chart of the World at 1:1 million Scale (ESRI Shapefile format)
The database was developed in 1991/1992 and national boundaries reflect political reality as of that time.

http://www.maproom.psu.edu/dcw/
 

GTOPO30 is a global digital elevation model (DEM) 

http://edc.usgs.gov/products/elevation/gtopo30/gtopo30.html

 

Another link

http://www.vdstech.com/map_data.htm 

USA Datasets:

U.S. State Borders (JSON):

http://ernestdelgado.com/elections/data_us.js

U.S. Census Data: 

http://www.esri.com/data/download/census2000_tigerline/index.html

Cartographic Boundary Files 

U.S.Census Bureau - TIGER/Line® 

 

Florida Data Blog:

http://www.gis-data.blogspot.com/ 

Texas Datasets:

GIS data for the state of Texas

http://www.tnris.org

 

New York State Data :

Cornell University Geospatial Information Repository
http://cugir.mannlib.cornell.edu/ 

Map Browser (Links to Shapefiles)

http://cugir.mannlib.cornell.edu/mapbrowse.jsp?series=counties

 

New York State Geographic Information Systems

http://www.nysgis.state.ny.us/

http://www.nysgis.state.ny.us/gisdata/

 

US States (contains some broken links)

http://library.stanford.edu/depts/gis/web.html

 

National Atlas of the USA 

http://www.nationalatlas.gov/maplayers.html 

http://www.nationalatlas.gov/atlasftp.html.
 

UK Datasets:

UK Administrative Geography:

http://www.statistics.gov.uk/geography/geographic_area_listings/administrative.asp

 

The GIS Digital Boundary Datasets held by English Nature

http://www.english-nature.org.uk/pubs/gis/gis_register.asp

Please read the Terms and Conditions when downloading

 

UK Boundaries:

http://www.statistics.gov.uk/geography/boundaries.asp

 

OpenStreetMap is a project aimed squarely at creating and providing free geographic data

http://wiki.openstreetmap.org/index.php/Planet.osm

 

Magic is a UK Government Site with downloads for Sites of Special Scientific Interest polygons etc.

France Datasets: 

France Départements:

http://www.ign.fr/affiche_rubrique.asp?rbr_id=1761

these files are provided with coords in Lambert 93 more doc here (in french)

Utilities: 

A Perl module to handle the Douglas-Peucker algorithm along with Haversine distance, angle calculations and perpendicular distance to a line.
USNaviguide_Douglas_Peucker.pm 

 

Converters:

Resources: Data Converters