Rain gardens are gardens designed to look good AND help soak up rain water that drains off your roof, driveway, and yard and ordinarily would be running into a storm drain. They are planted with native wildflowers, ground cover, shrubs, and trees. Rain gardens are designed to hold a few inches or more of water that then slowly filters into the ground water instead of running off to a storm drain or local creek. As a general rule of thumb they allow about 30% more water to soak into the ground than a typical lawn. Besides helping to reduce flooding in developed areas, rain gardens also reduce the amount of pollutants such as fertilizers and herbicides that wash off yards and enter storm sewers and eventually into nearby streams.
Rain garden resources:
Rain Gardens Technical Design Manual (Rainscapes, Montgomery County, Maryland)
This is a very detailed manual for DIY rain gardens that includes site assessment, design, planting plans and construction details for rain gardens.
Rain Garden Manual of New Jersey (Rutgers University Extension)
Excellent guide on how to design and construct rain gardens with sample plans and plant lists.
Rain Garden Info Guide (University of Delaware Extension)
Concise handout on rain garden benefits, facts, design, and construction.
Landscape Plug Manual (North Creek Nurseries)
Excellent tool to help you select plants for different zones of your rain garden, from wet to mostly dry.