In general, you are ok drinking tap water in western and northern Europe, U.S., Canada, Australia, New Zealand, and Japan. However, even in the US, we see contaminated tap water. When in doubt, drink bottled drinks, hot drinks, and make sure you avoid ice cubes! Ice cubes are made with tap water, not boiled water and as they melt, you may as well just drink tap water.
You can buy water purification kits if you are travelling in areas with dubious water sources.