Somewhat tangential, but: I'd love to have a script that automatically downloads the FLV file for every YouTube video I've bookmarked in del.icio.us. My collection of videos is slowly leaking away due to takedown requests.
Here is the method:
1) Use del.icio.us API to extract your bookmarks linking to youtube (http://del.icio.us/help/api/)
2) For each entry in the list, call the downloader script on the url.
3) Optionally convert the video to a better format than FLV (http://www.catonmat.net/blog/converting-youtube-flvs-to-a-be...)
Easy :)