The Scale of the Universe 2
In The Hitchhiker's Guide to the Galaxy, Zaphod Beeblebrox is stuck in a machine called the Total Perspective Vortex, a torture device that grants him the knowledge of how he personally relates in scale to the entire universe. It's meant to make him go insane, you see. The Scale of the Universe 2, by Cary Huang and Michael Huang, is basically that machine in Flash form; it isn't really a game but it'll still blow your mind.
Much like the original Scale of the Universe, you control this toy by using the mouse-wheel or scroll bar to scale in and out. You can also click on various objects to read their amusing text descriptions; these are new to the sequel. You start at human scale and can scroll all the way down to theoretical concepts like strings or all the way up to the potential size of the universe. That's pretty big, and you have to scroll for a long, long, long time time to get back to human scale from there. It's kind of terrifying, honestly. Try not to think about it too much.
It won't last more than about twenty minutes, even if you count reading all of the object descriptions, but The Scale of the Universe 2 is still a quick way to instill some humility in the player. Or, if you're like Beeblebrox, reminding you that even if you're comparatively miniscule you're still the coolest thing in the known universe.