Out of the box, the iPod is designed to work seamlessly with iTunes, so the system automatically syncs (by default) when you connect your iPod. This is typically fine for most users, but what about those times when you don't want this tag team to kick into gear for one reason or another? Perhaps you just had a library goof-up, and you need to use iPodRip to recover some playlists or media files before iTunes syncs and blows either away, or perhaps you're just in a hurry and don't have time to sync down that new 1.5GB movie you just bought, though you you still need to dump some files over for work or class. Whatever the scenario, there are a couple of simple ways to arbitrarily prevent iTunes from working its magic on a case-by-case basis.
The first method, I feel, is the least intrusive and most useful: assuming iTunes is running, all you need to do is hold down cmd+opt (possibly ctrl+alt on Windows, though I don't have a machine to test that on) when connecting your iPod to make iTunes look the other way and not begin a sync. The iPod should mount on your desktop and even appear in iTunes, but it simply won't sync. You're free to go about your business and eject the iPod as quickly as it mounted, with no argument from iTunes.
The second method assumes that you have enabled hard drive use for your iPod, and you don't have iTunes set to automatically start when you plug the 'pod in. In this case, simply quitting iTunes when plugging in your iPod will solve your problem altogether. Simply do your business and eject to get on your way quickly.