R-bloggers: the nexus of all things R.
R Views: an R community blog edited by RStudio.
Hadley Wickham: no introduction necessary.
Spatial Data and GIS
CRAN Task View: Analysis of Spatial Data. CRAN page with links to all tools CRAN has for working with spatial data.
GADM: Global Administrative Areas Database. The goal is to map the administrative areas of all countries, at all levels of sub-division at high spatial resolution. Very R-friendly: stores data as .rds files formatted for sf and sp packages, in the Geopackage (.gpkg) format, as shapefiles (.shp), and as KMZ files.
Canadian GIS: a collection of Canadian GIS and geospatial resources.
Statistics Canada 2016 Census Boundary files: Statistics Canada official geospatial data repository.
Geocomputation with R: an excellent (and very up-to-date) textbook on using R to work with geospatial data. Has a freely available online version and a print version.
r-spatial.org: a website and blog about using R to analyze spatial and spatio-temporal data.
Bounding Box Tool: just what it says on the tin. Can be very handy when you are making maps and need coordinates for your bounding box.