We go the last 2 weeks in Aug. Local kids are back in school. less crowded. It's Value season, so prices are much lower, and discounts come out, such as free dining, or 30 - 40% off room rates depending.
It is hot, but the water parks are great. We are usually there for 10 days. So can tour leisurely. We go to the parks at opening, then come back to our resort for a mid day break, swim. Then go back to a park at night with EMH.
Yes, it is rainy season, but it's generally an after noon rain, that doesn't last very long. It can rain in one park and not another. We just carry ponchos just in case. In Aug 08 we got caught in Toropical Storm Fay, and did get alot of rain, but we've been several times in Aug, and that was the worst one.
Mousesavers.com has a chart of Historical Discounts, and a chart of the WDW rate seasons for their resorts. Great info!