go-geojson provides a fluent Go API for building and marshalling GeoJSON objects.
This library was written as an alternative to github.com/paulmach/go.geojson, which isn't as nice to use when creating objects (e.g., one can't fluently set properties when creating a feature, it doesn't guide the user as to how to create positions or how many positions need to be in a line string or linear ring, etc.). github.com/paulmach/go.geojson currently supports more features though, in particular unmarshaling and bounding boxes.
To get the library:
go get github.com/barnardb/go-geojson