AlphaPNG

AlphaPNG file format plug-in for PhotoShop lets to save semi-transparent images with semi-transparent tones and an alpha-channel in 8-bit PNG format. AlphaPNG lets you choose your ideal color palette (RGB or Greyscale) and automatically optimizes it using our True-Color algorithm to convert your images into high quality indexed color PNG images with optimal dithering and optimized palettes. Save semi-transparent images in 8-bit PNG format. True-Color algorithm intelligently converts full-color images into indexed color.

Limitations:

no limitation