SMIL!
Found a cross-platform SMIL (Synchronized Multimedia Integration Language) player in AMBULENT.
The AMBULANT Open SMIL Player is an open-source, full SMIL 2.0 media player. It is intended for researchers and developers who want an source-code player upon which they can build higher-level systems solutions for authoring and content integration, or within which they can add new or extended support for networking and media transport components. The AMBULANT player may also be used as a complete, multi-platform media player for applications that do not need support for closed, proprietary media formats. The player is architected to support a range of SMIL 2.0 profiles (including desktop and mobile configurations) and runs under LINUX, Win32 and Macintosh OS X.
Also found a cross-platform SMIL authoring tool: SMILGen, written in Python.
SMILGen is a SMIL (and XML) authoring tool designed to ease the process of XML content creation. SMILGen understands XML syntax and handles the nesting and formatting XML. This allows authors to worry about the content that they are trying to author with out having to remember each quote and closing brace. SMILGen also understands the languages it authors, it knows what attributes a specific element uses or what child elements a given element may contain. Both of these features help eliminate a number of common XML syntax errors as well as making it easier to edit with out having a reference to the language right by your side.