GeoJSON is what geographic data looks like when it stops pretending not to be JSON. It is the lingua franca of web mapping - Leaflet, Mapbox, D3 and most spatial databases read it directly - and its one famous trap is coordinate order: longitude first, then latitude.
Every conversion below runs in this page, on your own machine. Nothing is uploaded, there is no queue, and there is no account to make. Pick a target format, or just drop a GEOJSON file above and choose afterwards.