One of the best things to have!! I completely agree with Dastank. I got a snowflake eel, and all my crabs mysteriously disappeared, but I wouldn't recommend getting one. They dig, and are known for knocking things over. Every morning I have to put things back where they belong. The bottle also works wonders for catching fish without any added stress. I have caught evry fish in my tank!!! Simply place a forozen cube in, hold it in the tank, and pull it out when you get what you want.
A few other inexpensive things I've picked up along the way are a turkey baster (great cleaning tool), a toothbrush, dental tools (you can get them at any 100Y store...go figure), a magnafying glass, and a long pair of tweezers (about 11" long, get them at makuman). 100Y stores are great!