Fling! local content onto your chromecast, no format restrictions!
Fling is a Java tool that allows you to stream local media from your computer to the TV. It ties into VLC, so you can transcode and play any video or audio format supported by VLC (which is basically everything). All this happens behind the scenes, the tool itself is incredibly simple.
I tried it, and it works brilliantly. It detects the Chromecast fast, then its a matter of simply dragging any video or audio file you want to play to the tool, and voila!
My computer was still open for me to use as I liked while the tool worked in the background. I still need to use this more to check how well it works and if there are frames being dropped, etc. but from the little time I've spent with it, this nifty little tool radically increases the usefulness of my Chromecast.
Best of all, since it doesn't use the Googlecast SDK, its open to use right away. If this trend continues, the Chromecast will soon be the most fully featured TV streamer in existence.