I finally got round to cleaning up my Django validator app. This app sends outgoing pages through an HTML validator and log any failures. I hadn't updated in ages, and it was quite broken.
It is called 'output-validator' not 'html-validator' because there is nothing specific to HTML - if you are generating other content types you can configure it to validate them, provided you have an external command line program that will do the validation.
For Ubuntu users, you should be aware of Bug 390604 which will kill the usefulness of validate from wdg-html-validator. The workaround is to install the wdg-html-validator from Debian stable. (It was this bug that meant I stopped using and updating my validator app in the first place).