I loves me some safety stuff, reflective tape on the helmet, bike, solar ankle light, too much orange clothing to mention, my kids are mortified when I turn on... but the single thing I wouldn't go without?
My LEDtronics flashing safety vest. What a huge leap in visibility. Pretty sure people would have to be blind to run into me (knock on wood). Had to buy more since I sewed one permanently onto my motorcycle jacket then gave other to girlfriend. Especially since we're often riding round Manhattan at night... it's a real comfort. http://www.ledtronics.com/Products/P...ls.aspx?WP=648
Second choice? The reflective tires. Another big leap forward.
So share yours, I'd love to pick up some more ideas.