This is not an endorsement. I do not accept payment for showing and discussing the tools I use, although I am very opinionated about them. For example, Apple products over Microsoft products. Having spent about three decades struggling with jinky Microsoft hackjobs with little personal joy to show for it, a decade ago I stopped that cycle of madness and switched to using Apple products and have experienced significant levels of mounting pleasure ever since. Likewise occurred when I switched from Martin guitars to Larrivee. I liked Martins okay, but I absolutely love to play a Larrivee. And on the subject of musical tools, these are latest additions to my meager kit: a Dr. Duck's theory wheel and a pair of Jetslides:
Never one to memorize the circle of fifths straight up or by mnemonic devices (something about fat cats eating bacon and big elephants driving something), circle of fourths, and all the related harmony and melody stuff tied to key signatures, I always just did the music math in my head when I had to. Never got fast at it, though, which wasn't a problem as long as I wasn't a session player...which I never was. Then I found this comprehensive music theory wheel and keep it handy at all times to quickly dial up key signature, harmonies and notes.
The Jetslides (one steel and the other brass) are an ingenious invention to quickly switch between playing fretted-note and slide guitar and back again with ease. I rarely bothered to play much slide until I found this dandy little tool. Jetslide makes them with glass sleeves too. I may someday get one of those, although I'm certain I will eventually drop and break it.