Under $20 - Magnetic pin cushion. I love the ease of this. I can just drop the pins in the general direction as I pull them out, and the magnet pulls them right in. I also have one for my wrist, which I think is pretty essential.
Over $20-- I *really* like Creative Grids rulers. They are textured on the back so that they don't slip on the fabric as you're cutting. I wasn't sure if I would like that, but WOW. They are fabulous!
Those are my two favorite tools as of this moment. :-) I also like Clover chalk wheels, and as everyone has said, a good cutting mat and rotary cutter.