A rain garden designed as a dry stream, fed from a downpipe
Rain gardens are at the forefront of a systhesis of landscape design and delivery of "eco-system services". Landscapes that capture and hold rainwater, allowing it to infiltrate into the ground can substantially reduce the risk of flash-flooding further along the drainage route. Not only that, they allow for a artistic delivery of landscapes which truely come alive when it rains. Commercially, a lot more can be done with them than I have seen so far.
A permeable driveway with adjacent planting in infiltration beds
Permeable landscapes are not just about swales. Where hard surfaces are required, these can and should be permeable, including the sub-base layers (permeable paving in front gardens is now UK law). Water soaking into the ground gives short and medium term storage of rainwater which is then available for take-up and evapotranspiration by nearby plants, creating healthier air and completing the local hydrological water cycle. As urban populations continue to grow globally and weather patterns become more unpredictable, existing infrastructure in rainy climates is clearly struggling to cope. Rain gardens, utilised on a wide scale, are a part of the solution.