Most useful tools/resources... in a general sense, not related to any specific instrument or type of music. The main ones that come to mind are:

My ears
Hearing something and recognizing what I'm listening to in a musical way (the product of ear training). The most important tool... listening and making sense of what I hear.

Open Ears & Mind
Kind of goes with the above. Listening to different kinds of music and different musicians playing different instruments. For example, I play guitar but I can learn something from Coleman Hawkins or Charlie Parker or Roy Eldridge. I don't play bluegrass and don't care for the sound of a banjo but I can still maybe learn something from Earl Scruggs. Heavy metal ain't my musical cup o tea, but some of the techniques and tools are kind of interesting and might be useful to me in a modified form.... maybe.

Reading Music
Learned to read music when I was 10. If a 10 year old can do it anyone can. Reading music isn't required but being able to opens up a lot of resources.

A Metronome
Few of us are born with the ability to keep a steady, specific tempo. Gotta learn somehow. Using a simple metronome does the trick.

A Keyboard
I play guitar but when I studied music theory I found it much easier to visualize and understand if I used a keyboard or piano. For example, 1-3-5 vs 1-b3-5 vs 1-3-5-b7 are easier for me to see and understand on a keyboard. In fact, any music theory... scales, chords, intervals, progressions, etc... was easier for me to understand and apply on a keyboard. Also helps a great deal with ear training.