When you sync your iPod/iPhone in iTunes 9 and you've got space left, check off the "Automatically fill free space with songs" box on the Music tab to use every last free megabyte. As David Chartier at The Finer Things in Mac site notes, there's no way to see what songs iTunes pulls and adds there for you. I'd like to think it uses some sort of smarts to determine what songs you haven't synced that you should have. Either way, nifty feature for quickly filling up your player without having to check off playlists manually.
Note: As Smarterware reader Fran points out, iPhone and iPod touch users may want to skip this if they're apt to install a lot of apps on the go, since it could mean there's no space left over for a new app.
Fill Every Last Megabyte of Your iPod's Free Space [Smarterware]