Are there any open source sprite format?

PNG just plainly sucks. I has no support for animations, timing or delta compression and sequence names. Also, PNG is slow at random access (you have to decompress image before use). Proprietary formats (like Diablo 2 DCC) are much better, but highly specialized for their games: some have no true color, other have no directions.

>>99
LZW uses variable length codes. Anything that doesn't use them will trounce it in speed.
Anything that doesnt achieves far worse compression ratio. Still they are faster than huffman codes and bit-shifting isnt an expensive operation.

Outside GIF, basically nobody uses LZW.
Because LZW was patented and open source crowd makes a drama from this.