Geography

Literally everything on Earth.

— Journals —

ACME - "international journal for critical geographies"

— Geographic information systems (GIS) —

Critical GIS

Cartographic Perspectives - open journal about cartography and other geographic visualization (including, but not limited to, GIS)

CriticalGIS.com - reading list and other resources on critical GIS

Learning GIS

Esri Academy - Esri's MOOCs on using its ArcGIS software, some of which are free

QGIS Tutorials and Tips - exactly what it sounds like

Spatial Thoughts - "learning platform for modern geospatial technologies"

Open-source

Free GIS Datasets - exactly what it sounds like

GRASS GIS - free GIS software (with lessons on how to use it)

OSGeo - website of the Open Source Geospatial Foundation

QGIS - similar to GRASS GIS

— Transport geography —

The Geography of Transport Systems - online version of an educational book by Jean-Paul Rodrigue

TransitWiki - wiki by and for transit professionals, set up by UCLA's Institute of Transportation Studies