Ask HN: What are your best hacker tips/tricks/tools?

Hey HN Community,

What are some of your favorite/most useful hacking tools you've picked up over the years? I mean "hacking" a little less in the computer security sense and a little more in the creative problem solving sense. But I'll be happy with either.

For example, about a year ago I learned how to use charles proxy with a self signed ssl cert to view all the network activity coming in and out my phone and computer. This was awesome because then I could make api calls directly to whatever I wanted.

Previous to that I'd learned how to decompile and recompile android APK's. One of the funner projects I've done in a while was decompiling an app, installing my own ssl cert (via the android manifest), recompiling and looking at the network traffic directly using the proxy.

I want to learn more things like this. What are some tools/tricks you've picked up over the years where you sat back and thought "Woah... that was cool"?