The first ten minutes of a Valheim run are the most expensive ones. You pick a seed, you spawn, and only then do you find out whether the starting island has the resources you wanted. By that point you have already committed.
So I built a small tool that draws the map before you commit. Type a seed, get the map. That part is not novel — there are several seed viewers around. The part I actually wanted was different: reading the seed out of a save you already have.
If you have a world you have been playing for months and you no longer remember the seed, the world folder contains it. Drop the folder in and the tool reads the exact seed, then draws the map for it. It handles both .fwl files from before 1.0 and _main.N.fwl2 from 1.0. It also lists what it can find in the save — dungeons, altars, traders — so you can see what is left to visit.
Two decisions worth mentioning:
Nothing is uploaded. The save file is parsed in the browser; the file never leaves your machine. A world save is a personal thing and I did not want to be in the position of storing anyone's.
No account, no install. It is a static page. Open it, use it, close it.
Honest limits: it is a viewer, not an editor. It will not modify your world, and it cannot tell you whether a seed is "good" — that depends entirely on what you are trying to do (a speedrun start and a base-building start want opposite things).
It is free and it will stay free: https://valheimworldgenerator.com