pngquant Photoshop plug-in
It provides PNG saving better than Save for Web. Typically creates PNG files 60-70% smaller than 24/32-bit PNG from Save for Web, and unlike 8-bit PNG from Photoshop, always supports semi-transparent alpha channel.
Download for Mac OS X and copy
/Applications/Adobe Photoshop C*/Plug-ins/.
Download for Windows (SuperPNG plug-in)
To use it choose regular Save As and PNG for Web (pngquant) as the file type. A small web-optimized PNG file will be written without any prompts.