-
Color Brewer: Tool created by Cynthia Brewer that offers advice for using color on maps, specifically with thematic mapping. Lets you export color schemes to various formats.
-
Mapshaper for generalizing your geospatial data which helps your maps not only look better but load faster. Mapshaper let's you preview how generalized it looks before you export it.
-
Nathaniel Kelso's Geo How To wiki though now a little out dated still has lots of great info in it.
-
Tom Patterson's shaded relief for web mapping guide. Helpful for those interested in using terrain data for cartography on the web.
-
Thematic Cartography Guide by Axis maps; "Our goal is to cover the important concepts in cartography and flag the important decision points in the map-making process."
-
CartoTalk's tutorials one of the forums by the North American Cartographic Information Society's (NACIS) website.
-
Cartographic Perspectives an open access journal of the North American Cartographic Information Society that features tutorials and academic articles relating to cartography.
-
Let's Make a Map Mike Bostock's (creator of D3.JS) tutorial on making a map using TopoJSON and D3.JS. There's also one for making a bubble map. Bl.ocks are great places to learn about D3 btw.
-
Tile Mill guides though MapBox no longer uses CartoCSS for their products, the Tile Mill docs still are a great resource if you're using it in other places or want to export "slippy map" tiles from data a la carte in a raster format.
-
Labeling and Text Hierarchy in Cartography by Axis Maps.
-
Stamen's checklist for (web)maps solid UX principles to strive for when creating an interactive web map.
IMO the best way to learn is from looking carefully at what others have done with map design; spefically in regards to color, typography, line work, etc. Try to replicate it in your own work or use it as a starting point.
-
National Geographic some of the best print maps.
-
GreenInfo Network A GIS and Cartography non-profit that does work for the public interest.
-
Stamen Design an early innovator in cartography and data visualization for the web.
-
Mamata Akella formerly of NPS maps, now at CartoDB making lots of great maps & tutorials.
-
Axis Maps is a map design studio run by Andy Woodruff and others. They have a great blog and made some really cool Typographic Maps.
-
Daniel Huffman independent cartographer, fellow at UW Madison Cartography lab. He also has a blog on map fopauxes called Cartastrophe
-
Tom Patterson of the National Park Service maps team.
Articles, etc.
-
The Dirty Little Secret Data Journalists Aren't Telling You or why it's important to choose your color scheme and bins wisely... by the Washington Post.
-
Is Your Map Effective?: Map evaluation guidelines, principles of good design.
Websites, etc.
- Cartotalk home of the North American Cartographic Information Society for all things maps, including a forum where you can ask for advice on map design from friendly, seasoned industry professionals, even veteran cartographers.