Shameless plug: check out jsmpeg[1] - it's an MPEG1 decoder written in JavaScript that's capable of low latency streaming via WebSockets, weighs only ~25kb gzipped, offloads part of the decoding to the GPU via WebGL and runs smoothly on an iPhone4.
Of course it has some drawbacks, as MPEG1 is a pretty old codec. The data rate is fairly high and it struggles with high resolution streams. 720p still decodes nicely in realtime on most mobile devices, though.