OSM is love. OSM is live.

Working with the data

https://overpass-turbo.eu/
Query OSM data. For simple things the wizard is really easy to use.

https://umap.openstreetmap.fr/
Create custom maps with OSM data e.g. for websites or data visualization.

https://resultmaps.neis-one.org/
Various cool visualizations and heat maps.

Editing tools

https://streetcomplete.app/
Shows missing data as “quests” on a map. Also includes more and more general purpose editing functionality via overlays.

https://vespucci.io/
If more elaborate editing is required on the go.

https://opentracksapp.com/
Actually a sports tracker, but you can also upload not-so-sporty tracks to OSM or use them as overlay in an editor. It’s also possible to add markers with text or images for later inspection.

https://ideditor.com/
When my smartphone screen just isn’t enough.

https://taginfo.openstreetmap.org/keys
This has statistics on key and value utilization to better understand how frequently some tags are used.

Navigation apps

https://organicmaps.app/
All-round navigation/map app.

https://osmand.net/
Very versatile, but a little bit more complex. Can be highly customized e.g. to show street lighting on the map.

https://www.magicearth.com/
For car navigation including traffic data. It’s not open source, but in return it has turn-by-turn voice instructions without needing Google Play services.

Honorable mentions

https://wiki.openstreetmap.org/
It has even more information on everything listed on here and obviously many other topics.

https://matrix.to/#/#osm-de:matrix.org
The people there know more than me so I just ask them things from time to time.

https://lubiland.de/
My personal website :)