A few things I've built and use.
A few tools I’ve built, mostly for my own use.
Visualise multiple GPS tracks at once on an interactive map. Useful for getting a picture of where you’ve been.
gpxheatmap.rkan.org — Svelte / Leaflet
Theoretical UV index (clear-sky) for any location and date. No cloud cover taken into account — just the sun geometry. Useful for planning outdoor activities.