Like I said, we've all seen them.
None are identical, but most have something in common. Namely, that peculiar ringtone. We call it peculiar because it isn't actually included with the iPhone. Luckily, most of us have it installed on our computers already.
As LifeClever points out, those with iLife '08 installed can find the ringtone in
- /Library/Audio/Apple Loops/Apple/iLife Sound Effects/Work - Home/Cell Phone Ringing.aif.
Finally, select "Send ringtone to iTunes" from the "Share" menu and you're all set. If that seems like too much work for you, just download the file as a ringtone here. Incidentally, there are a lot more sound files in Loops directories that would make good ringtones as well.