If you're desperate to save space (or creating thumbnails), Windows/C# devs can lossily cut down PNGs to 256 color/8-bit using nQuant http://nquant.codeplex.com/

Matt Wrock documented the creation of the tool, including details on the approaches taken by tools like OptiPNG: http://www.mattwrock.com/post/2011/09/05/Convert-32-bit-PNGs...