You don't even need a computer, just grab your iPhone and get to work. For reference, you don't need to set up custom ringtones using your iPhone.

I've just gotten a new Mac and want to transfer my music onto it, but I don't have an easy way to get it off my old computer. Is there a simple, free way I can just transfer it from my iPod or ...

Plug your iPhone into your computer. Click on it in the iTunes sidebar and go to the "Music" tab. Check the "Sync Music" box and it'll re-sync all or your music and playlists.