1) THREAD HEAVEN. It costs between $3-$5 and comes in a little tiny blue plastic box. It is basically a little square of glycerin, and it's useful for smoothing stubborn threads like rayon or metallics.
2) CURVED EMBROIDERY SCISSORS - These are a pricier version, but Gingher are the best! I have a cheap pair of orange-handled Fiskars and they do the job too. The curved scissors are perfect for snipping the small ends of threads RIGHT up next to the fabric. These are especially useful for any kind of apparel since you don't want even tiny thread hanging off the back.
3) TEAR AWAY STABILIZER - A must for stitching on Onesies, t-shirts, or very thinly woven fabrics.
4) DISAPPEARING INK MARKING PEN - The blue side comes out with water; the purple side will eventually disappear by itself, but water works on it as well. This is the perfect tool to touch up patterns or create your own patterns on light colored fabrics.
5) EMERY PIN CUSHION - These come alone or are often found as a little "strawberry" next to the tomato on a traditional tomato pin cushion. The emery material inside sharpens needles. It's important to keep your needles sharp to avoid ruining your fabric or threads. Also, sharp needles pass through fabric more quickly.
6) A CUTE PIN CUSHION - Because you can never have too many cute pin cushions.