iSpring 10 in. x 4.5 in Heavy Duty 2-Stage Undersink Water Filtration System $164.89 + Free Shipping

If you're really wanting to improve the quality of your drinking water, look at getting a full Reverse osmosis system. There are several that you can get that are not much more than this 2 stage system.
Normally I wouldn't jump into someone's posted deal to mention this, but I live in an area that has sub-par city water and have done an absolute ton of research on this.

For those that are interested the system i'm referring to is made by APEC, the model number is ROES-50, and the cost starts at $190. The system goes under the sink just like the one from this deal but is a FAR better filtration system and produces something that is actually a pleasure to drink.

Reverse Osmosis water isn't particularly good for you though as the process strips all the minerals out of the water. It's not that bad people drink it all the time but there are some studies showing harmful effects. The best IMO is this type of filter with a coconut charcoal filter for chloramine and pesticides and a bone char charcoal filter for heavy metals.