1) To make widgets accessible to all users, place them in HD/Library/Widgets as opposed to ~/Library/Widgets
2) When installing screen savers there should be an option to install for all users or not (you will need an admin password)
3) For iTunes, you can link music libraries without having two copies on your hard drive. First make sure that the users Library that you want shared, is in a shared folder thats accessible to other users. Then in iTunes go to Preferences>Advanced and under and click change, then navigate to the other users, itunes library. Make sure to deselect the box that says 'Copy files to iTunes Music folder when adding to library'.
BTW, I don't know if thats the best way to do it, thats just the way that I do it, and it works fine for me.