When you think about it, the most important ingredient in beer is actually the ingredient that many home brewers overlook: water. Water is the most prevalent ingredient in beer and has a significant impact on the taste, pH, starch conversion, and overall quality of a beer. One of the easiest and lowest cost methods for making a significant impact on the quality of your home brewed beer is to simply filter your water with a carbon filter or reverse osmosis system. A carbon filter will dramatically reduce the amount of chlorine in your brewing water; chlorine can be responsible for creating off flavors in your beer, and it is important to remove it. Additionally, a good carbon filter will remove a host of other potential contaminants including benzene, radon, pesticides, herbicides, and foul odors from water.
The carbon filter that I use can be found here:
Below is a photo of the carbon filtering kit: