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What are the best worldbuilding tools you currently use?
(reddthat.com)
I rely pretty heavily on Mythic Game Master Emulator and it's methodology. The long and short of it is that you question assumptions, set the odds of your assumption, then roll dice. The result can be "yes", "no", or in very high or low rolls, "exceptionally yes" or "exceptionally no". Can introduce some fun surprises. I use it whenever I get stuck on something and need to inject some randomness.
It also includes a huge number of random lists for creating characters, events, locations, etc. Really any list and some dice can help with inspiration, dice rolls are a powerful tool.