"Podcast Generator is a free web based podcast publishing script written in PHP: upload media files (audio-video) via a web form along with episode information and automatically create podcast w3c-compliant feeds including iTunes specific tags. It also features a comprehensive web administration [backend]."

Podcastamatic is an open source, Perl-based web application enabling you to distribute your music. "Podcastamatic automates the creation of a webpage and a RSS feed for your podcast, based on MP3 tags. A webpage can either be automatically generated from scratch or generated from a template you create." The system can work with MP4 files as well as MP3 and iTunes tags are also supported (even including images). There are Podcastamatic forums too.

Note: the above link doesn't seem to be working at the moment, see my comment for an alternative download location.

"PodAdmin is a very simple web-based podcast manager. Upload your new episode via a web form, PodAdmin will then grab the title and description from the ID3 tags and generate a new RSS feed on your website. Optionally you can specify the title and description if it is not embeded in your audio file. Then all you need to do is link to the generated RSS file. PodAdmin uses PHP and MySQL and works with any content management system, or none at all."