want to download flv from youtube. try out the grease-monkey for firefox plugin or try the beta downtube.
greasemonkey - download the plugin, after you install the plugin your firefox browser will have monkey head on the bottom right of your firefox browser. After that get the script to download flv from the youtube browser.
Downtube - this is already made and it's kind userfriendly, however the command prompt is annoying, the script is an exact command from the *nix that is wget. it also have ffmpeg plugin to convert the flv to mpg so that you can share your download video with your friends. You need to execute the program downtube32 to use the downloader. After you execute the program will lunch a build-in browser client and there you go.. simply click your video and try to download it with the big button on the it's browser.
As for i prefer the greasemonkey plugin for firefox. I give greasemonkey 4 out of 5. Moreover i think the greasemonkey can customize and input script to download the radioblogclubs rbs files.
