There are a number available. I am about to take the plunge and subscribe to one for offline use. I would prefer to only use one and will use it in the Alps and Svalbard. Recommendations please.
John - just posted this on your Facebook question:
if you're Android, the best by far is http://www.alpinequest.net : free version to test, but small one off payment for fully featured with loads of map sources (even access free OS & French IGN etc if you know how ?)
Maps can be viewed whilst online then saved for offline use - you define the area to save and zoom scale required
I did a quick search and loaded Svalbard mapping into AQ from https://toposvalbard.npolar.no/tsmini.html
Below is example of this Svalbard map loaded into Alpine Quest
I use
https://www.locusmap.app/
I think locusmap 3 classic is the best option, locusmap 4 is a cut down subscription version both versions are actively updated.
its £8.99 you can download vector maps and a wide range of maps (swiss topo, IGN.