I’ve tested a few addons for Firefox and a couple of extensions for Chrome. The only thing that seems to work right every time I use it is “1-Click YouTube Video Downloader” for Firefox.
I’m sure there are other methods but I haven’t explored any other than what I just mentioned. It’s rare that I need to download a video from any streaming service, so it’s not high on my priority list to find the best way.
When you use “1-Click YouTube Video Downloader”, you get a list of file formats available for that particular video. I haven’t seen MP3 (audio only) on any list but I’ve seen MP4, WEBM, FLV and 3GP.
I suppose it depends on who you listen to. Downloading anything from the Internet isn’t inherently illegal. Copyright infringement only occurs when distributing (as in uploading and sharing). Certain entities (like the MPAA and RIAA) like to say it’s stealing if you don’t download something using an approved method.
Everything on the web is downloaded in one form or another, even streaming video. It’s how it all works. The issues arise when you try to share it online. Don’t do it and that’s all I can say about that.
Every PC and mobile device has a video player of some kind available. Sometimes one is installed from the beginning and sometimes you have to download something.
I like VLC. The versions that work or Windows, Linux and on my Android phone work very well for me. I’ve even watched full-length movies on my phone, without them sucking the battery dry on the rare occasions that I do.
Most of the videos I download are freely distributed documentaries. They’re boring enough that if I have to suffer from long pauses on YouTube, I probably wouldn’t continue to watch them.
Of course, you can now watch full movies on YouTube (distributed by YouTube), both free and commercial. It would really suck to be stuck watching a movie you paid for that pauses every couple of minutes. I don’t think the folks at Google (who owns YouTube) would have a major problem with people downloading the things they pay for. I’m just guessing, of course, because they may not like it at all.