For about the past six months or so, I have been using an aluminum water bottle from Klean Kanteen. I started using it because it holds more water than typical plastic bottle, thus fewer trips to the water cooler. It’s also much more sanitary than a plastic bottle (if you’re reusing it). Now for the best part; ounce for ounce, bottled water is more than DOUBLE the cost of gasoline! Lifehacker has a great write up on the cost savings of carrying a water bottle.