Crushpngs is a wrapper around pngcrush to help with handling a large number of files.
It provides a way to only work on files modified since a certain date and gives a brief summary of the savings. It also works around an unfortunate bug in some versions of pngcrush where the "crushed" file is sometimes larger than the original.
Crushpngs is Copyright 2009 - 2011 David Hoover and released under the GPL. The most recent version is available at http://github.com/deadmoose/crushpngs