What are some good places for July 4th fireworks in Southern Westchester?

The fireworks at Kensico Dam were a special treat as the dam itself is used as a backdrop and helps frame the fireworks. The town of Ossining puts on a really great show (typically on the 3rd to avoid competition on the 4th).
Also three churches in Ossining (and more in Westchester I suspect) have annual fairs during the summer with excellent firework displays.

Well, there are a ton of pretty darn nice displays all over Westchester, for sure, but I happen to think that the one held at the White Plains High School is pretty grand. It can get super duper uber crowded, however, and parking can be tough, so it is best to arrive early, bring a blanket and relax on the grassy hill. There are refreshments available as well as inflatable rides for the kiddies. Another great one is at the Kensico Dam Plaza in Valhalla. It has a wide grassy field perfect for chairs and blankets and evening picnics. The display is awesome and usually set to music by the Westchester Philharmonic. And last, but certainly not least, it is always a hoot to see the lavish display spraying over the Long Island Sound and Rye Beach at Playland Park. They also hold really nice weekly fireworks in case you can't make the big July 4th event.