Once it's cold enough, my favorite tool is the $20 floor scraper I got a couple years ago. If you're as fanatical about ice quality as I am, this heavy, telescoping glorified razor blade will shave off all the bumps and ridges in your rink, leaving you with a surface free of major bumps. Super important if you're having kids or rookie skaters over.
This is the model I have:
http://www.google.com/products/catalog? ... J8BEPMCMAQ
Got it at Home Depot in the flooring section.
What's your favorite rink tool?