Testcards are images (or video loops) used to test and calibrate video equipment; if you live in the UK you've probably seen Test Card F at some point (even if it's only on /Life on Mars/). While some organisations have made images of video testcards available online, they're typically in limited resolution or odd formats.
It shouldn't be too hard to write a library for generating high-quality testcard images in various formats that could be used by video software -- for example, MPlayer could have a testcard input module that could be used for adjusting your monitor or testing video encoders. I'd expect it to support at least 100%/95%/75% colour bars, pulse and bar, convergence grid and PLUGE; more complex patterns like TCF and PM5544 should also be possible. Since the software would be generating the images on the fly, it would be possible to regenerate them for different video standards, adjusting features (such as frequency gratings) as appropriate.
A similar library could be provided for overlaying timecode (or subtitles) on a video signal.
TCF and the newer Test Card W are certainly under BBC copyright, and thus can't be used directly; it would be nice to have an "open testcard" that offered similar features but was available under a Creative Commons license.
Andy Burns wrote to point me at the Test Card Maker software which renders testcards from textual descriptions; it'd be a good starting point for the library.