Stuff I find really helpful for working with map data

Note: this is not exhaustive, and will probably be updated more as needed

• ogr2ogr: part of GDAL, great for converting geodata into different formats
• gpsbabel: like ogr2ogr, useful for converting geodata into other formats;
• geojson.io: made by Tom MacWright of Mapbox, basically just a really useful tool for taking a quick glance at your data and seeing what's there before getting into design for your own map
• togeojson: another useful Mapbox-built tool for converting geodata when ogr2ogr is too onerous
• joiner: made by my friend Michael Keller; super useful for combinging existing geojson with other json data
• geojson-merge: got a bunch of geojson files that you'd rather put into one file? PROBLEM SOLVED, MY FRIEND.
• a pile of shell scripts by my friend Chris Henrick: just a way of making GDAL stuff slightly less onerous.

And a list of other people's lists:

• Tom MacWright's awesome geojson list of tools for working with geojson data

• Maptime's lessons and resources