(Pronounced "ming", MIME type video/x-mng)
Official home page: http://www.libpng.org/pub/mng/
MNG (Multiple-image Network Graphics) is a "PNG-like image format supporting multiple images, animation and transparent JPEGs." (quote from their web page)
PNG is a single-image format. GIF, then, supported some sort of animations from the beginning (even when this fact wasn't known at the beginning).
MNG is an extension to PNG, designed since summer 1996 and froze to "1.0 status" in January 11, 2001.
GIF animations, as implied before, sort of suck. GIF supports only simple animation types. MNG, then, offers goodies like objects and sprites - no need to copy your images, you can move cut-outs. It also supports nested loops (GIF could only loop the whole sequence).
It compresses better than GIF, too (by virtue of PNG format).