RIOT can do that, and a lot more.
Radical Image Optimization Tool (RIOT for short) is a free image optimizer that will let you to visually adjust compression parameters while keeping minimum file size.
It uses with a side by side (dual view) or single view interface to compare the original with the optimized image in real time and instantly see the resulting file size.
The image optimizer is lightweight, fast and simple to use, yet powerful for advanced users. You will be able to control compression, number of colours, meta data settings and much more, and select image format (JPEG, GIF or PNG) for your output file.
Another possibility is to use a combination of pngquant, pngout, and pngcrush, as described here, but this is from the command line.
Here are the instructions how to convert png24 images into png8 for ie6 goodness, all through the command line using open source tools (i think) pngquant +pngout + pngcrush.
1- quantize image into 256 (so basically png8′s look crap with large sprites or sprites with a large colour range).
pngquant 256 some_24_bit.png
2- convert image form a png24 to a png8
pngout -c3 -d8 -y -force some_24_bit.png some_8_bit.png
3- compress image
pngcrush some_8_bit.png -bit_depth 8 -brute -rem alla -reduce some_8_bit_small.png