Mapshaper is a great open source tool and online resource for simplifying maps and boundaries and working with geojson, topojson and other formats.
It's pretty simple but some of the usage of its command line features could be clearer or isn't necessarily obvious or intuitive. I've been adding a few self-answer questions here when I've encountered something roundabout and asked a couple of questions when stuck.
There isn't a tag for it and I don't yet have the rep to create one.
Can we have one?