Good thing I clicked on TipNut.com before renting a power washer to clean our concrete patio and pool deck. There's more than a year's worth of mildew and/or algae buildup that has spread around the concrete surface. I remember that my grandmother's front steps looked like this.
Tim Carter has a succint little video on TipNut.com over on the right side as the featured video that shows you exactly how to clean that up and have your patio looking as good as new for summer grillin'.
Here's what you need to get started:
Oxygen bleach (not chlorine bleach)
Heavy Duty Push Broom