The Shockwave Flash File Format. Compact format, and looks amazing (at least until your browser crashes the first time =)
It's one of the most widely used vector-based animation file formats in the web.
Macromedia has released SDKs for this format, and complete file format specifications. However, there aren't many 3rd party programs that would support SWF without using the proprietary SDK, because, according to the developers, the format is pain to work with (for example, the original spec had gaping holes).
Nevertheless, GPLed player and even a Perl SWF creation module exist...
Official home page of the SDK, SWF specs and list of third-party tools: http://www.openswf.org/