1. Pyramids (I don't care what 'qualifications' people want to put on this list, but considering they're still standing, it has to be one of the greatest engineering feats of all time. That said, I don't think the Great Wall of China should be on the list, because it's only a marvel due to its sheer length...it didn't take an Archimedes or an alien to build it).
The rest are in no particular order:
2. The Big Dig (the I-93 Construction Project in Boston). Simply unbelievable. Yeah, it's a huge waste of taxpayers dollars at this point (way over budget), but the fact that they're able to do it boggles the mind.
3. The Space Program. I'm including everything from Apollo to Voyager to Mir to Hubble to the International Space Station in this one, because each builds off the last. The remnants of humanity will one day exist only in space-borne objects.
4. The Course of Empire, by Thomas Cole. Five Paintings that completely and decisively sum up the whole of our existence. Buried in relative obscurity in the New-York Historical Society (77th and Central Park West), although you can see them on the web. Of course, seeing them in person is breathtaking.
5. The complete works of Beethoven. It seems incomprehensible to me that one man could dream up so much beauty, yet he did. (Feel free to substitute a favorite composer of yours in this item, or the more literary-minded might substitute Shakespeare)
6. The Titanic. Supposedly the height of mankind's ingenuity and skill...instead, the victim of its pride. Tells us more about us than we really want to know.
7. The number 'i'. It doesn't exist; it's an imaginary number. Yet not only did we determine it's existence, it is an invaluable part of our mathematics. Profundity and absurdity.