PNG
People just don't use PNG as much as GIFs because of browser incompatability.
only later versions of IE and Netscape support PNG. Gifs can do transparencies
too, so most people just use GIFs, even if PNG's may be higher quality images
Something else new: SVG (Scalable Vector Graphics): they use XML, code is used to draw vector images it is written right into your html and may one day replace or challeng the use of Flash vector graphics.
And also: SMIL (Synchronized Multimedia Interface language): A mark-up language like html that will make motion graphics and rollovers, coming soon.