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.

>>91
Is this a serious question?

LZW uses variable length codes. Anything that doesn't use them will trounce it in speed. Five seconds in Google: http://www.quicklz.com/

Of my personal knowledge, recent versions Windows use similar stuff for compression in the kernel (for NTFS, cache files, and the hibernation file) and for some userspace components (remote desktop protocol and a few others). Basically you have a bitmask every once in a while telling you which bytes are literals and which ones are matches.

Hint: Outside GIF, basically nobody uses LZW. What does that tell you about its compression/speed tradeoff? It has the performance disadvantages of variable length encoding without its main adaptability benefit.