This website offers advanced PNG files compression. TinyPNG uses smart lossy compression techniques to reduce the file size of your PNG files. By selectively decreasing the number of colours in the image, fewer bytes are required to store the data. The effect is nearly invisible but it makes a very large difference in file size.
TinyPNG will make your web page load faster. I used it few times and was pleased with results.
#1
If you want an “off-line” version of TinyPNG, see pngquant or ImageAlpha (all of them use the same method).
#2
One more
http://media4x.com/png-minimizer/