If your water smells like rotten eggs, it may be time to replace the anode rod found within your water heater. Water can contain anaerobic bacteria which reacts to the anode rod's magnesium and aluminum material. This reaction produces hydrogen sulfide gas or the classic "rotten eggs" smell. Anode rods can be purchased at most hardware stores and if you have questions on replacement, seek assistance from your local plumbing company. Plumbers will have all the necessary tools and expertise to complete the replacement or answer any questions that you have concerning your water heater.
Please note that this smell is not to be confused with a gas leak which also mimics a rotten egg smell. If you suspect that you may have a gas leak, immediately leave the premises, notify emergency responders at 911, and call Nicor Gas at 888-642-6748.