I would like to manipulate the content of a textfield/wygwam-field in expression-engine. For example, when someone uploads an mp3-file with the file-uploading-tool, I would like to convert the link into a flash-embed that calls an mp3-flashplayer.
Another example is that whenever someone posts a link to a vimeo-video I would like to convert it into an embeded version of that video.
You could create an extension that hangs on the entry_submission_ready hook that modifies the content in the appropriate fields prior to it being saved to the DB.