I take bottled drinking water to work - although Sydney tap water is of good quality, I don't want to drink the tap water in my office because the building is an old converted factory building, and I'm not sure how good the pipes are. Having bought a couple of bottles of drinking water I now rotate the bottles and refill them each night with chilled filtered tap water. The filtered water costs a few cents a bottle, and the quality is just as good as your typical bottled water brand by all accounts.
If you want to impress your workmates you can spend it up on the initial purchase of bottled water and have "exclusive" name brand bottles for your home made bottled water ;)
I usually also pop half a tablet of aspirin into the bottle in the morning - I happen to like the taste of aspirin, don't have any stomach problems, and the 150mg a day of aspirin might be good for my health.