It's been just over 3 months since the International Digital Publishing Forum started building an archive of Epub3 and things are moving right along. The collection is still small, but as of today it includes all the major features if the new format.
You can download the Epub files and find everything from embedded audio and video to MathML and Arabic and Japanese text (demonstrates RTL and TTB font directions). There's even a demo Epub that tests the Canvas HTML5 tag. That is a simple demo but it's still interesting.
I also found the multiple tests of font rendering to be a good idea because it shows why the archive exists. This archive is intended less to show of features to readers as it is to give developers a set of demo files to test their rendering engine. This archive saves developers the effort of writing their own test files, and as a corollary it increases the likely hood that the all the requirements of the spec will be implemented by the developer.
And that last part is important; I have a couple different ereaders in my collection which don't support all the parts of the current Epub spec. I'd really like to see that kind of sloppyness end.
You can find the files here. You'll need either Readium, Azardi, and the latest version of iBooks to see the features. I'd go with Readium, even though it requires Chrome. I already noticed that iBooks doesn't support at least one of the new features.