My most-useful spinning tool after my spindles and wheel is my ballwinder! and a library card :-) if you're in a reasonably large town, they're likely to have at least some spinning books, and maybe really good ones. Mine has Paula Simmon's 'Spinning for Softness and Speed', and Connie Delaney's 'Spindle Spinning: From Novice to Expert'. Great resources! And they did get Aldon Amos' book once it came out, too. My favorite author is Mabel Ross, so most of her books (ok, all!) are on my bookshelf.
So, my ballwinder is my favorite tool, along with my niddy-noddy; then my swift. In a pinch I'll use a nostepinne -- the ballwinder is basically a nostepinne with a built-in crank. All that said, there are "human" or inexpensive equivalents. The thumb makes a great nostepinne (just don't wind too tight!), the fore-arm is about a 2-foot-skein niddy, and the lower leg (wind over your knee and under your foot -- shoes off! -- with knee bent, while sitting) is a 4-foot-skein niddy or there-abouts.
(posted by me on lj/spinningfiber, 15apr07)
So, what's your most useful spinning tool? Add it with a comment below.