dfGallery 2.0 is one of the coolest Free Flash Galleries with 200,000+ downloads of v1.0 from their official website.It has an amazing theme engine built to support custom themes with multiple skins for each. The administration console built on CodeIginter lets you manage multiple galleries and albums of different types.

SimpleViewer is a free, customizable Flash image viewing application. This flash gallery is only 17kb and can be customized easily to suit your needs.