Realistic samples of test files that can be distributed under appropriate licenses are a really useful resource when building preservation tools. This page hosts some information about some of the available corpora of digital items.
This page holds links to corpora or lists of corpora - if any are of particular interest they should have child wiki pages dedicated to them.
* Much of NASAs data and images are freely re-usable, see e.g. [http://earthobservatory.nasa.gov/ImageUse/] * Andy Jackson tags such resources as 'corpora' on delicious. See [http://www.delicious.com/beardedstoat/corpora]