By hooking up your downspout to a rain barrel, you can improve water quality in Little Hunting Creek (and the Chesapeake Bay).
By capturing the first inch of rain falling on your roof, a rain barrel reduces the volume of stormwater (and the pollutants it carries) flowing from your property. It releases it slowly, allowing it to soak in rather than running down the storm drain and into the creek.
By attaching a hose to the spigot at the bottom, you have a handy source of free water. It's better for your plants because it is naturally soft!
The Northern VA Soil and Water Conservation District holds workshops for homeowners who want to build or buy a rain barrel (which are often made of recycled industrial pickle containers). Check their website.