via youtube.com
Ever notice this sound when you unlock something on Xbox Live? Isn't it amazing how satisfying it feels? Play it back a few times. For gamers, this is like hitting the little button in the cage that dispenses crack.
Now that you have access to this Youtube video, you can hit that crack button as many times as you want.
If you're creating a service or game online, think about the sounds that you can associate with peak experiences. Human beings are emotional creatures... the more we are aware of how these emotions can be associated with sounds, sights or actions, the better we can design things that are pleasurable and addictive.