Have you met or heard of these synchronizing troubles about iPods or iTunes?

1 You want to back up your iPod library when your computer's hard drive is damaged, but you can't.
2 You want to copy your iPod library from one computer to another by using your iPod, because you have two computers, one at home and another in office or somewhere else; but you can't.
3 You want to just add some your favorite songs from maybe other's PC to your iPod, but this is dangerous to lost all else.
4 You got two iPods and want to transfer songs and movies from old to new.