Many of my jaw dropping sites have already been mentioned:

Neuschwanstein
View of Florence from Piazalle Michelangelo
Sistine Chapel
Sagrada Familia
the mass graves at Dachau

A lot of mine would be repeats, but I'll add a couple I haven't seen much on this thread:

The first sighting of Denali (Mt. McKinley) peeking through the cloudtops
Topping the spiral staircase at Saint-Chappelle - still a WOW fresh in my mind
The cable car ride up to the Schilthorn after 1 meter of fresh snowfall
Lava entering the ocean at Kilauea

The Hermitage & ajoining Winter Palace in St. Petersburg, Russia. I'll never forget walking around the corner and seeing those brilliantly colored buildings standing before me. AWESOME!

When we arrived at our hotel in Ravello, Italy we opened the double doors and the view of the Bay of Salerno and the mountains was breathtaking. To top it off, there was a rainbow.

It was very interesting reading all of the postings on this subject. My wife and I have been fortunate enough to have seen many of them. While I was reading them I could not help but reflect on how fortunate we all are who have had these opportunities. Perhaps at this time of year with Christmas on the horizon it is appropriate to consider that.

The first European trip, Big Ben, The Pontcysyllte Aquaduct in Wales, The Eiffel Tower, The Venus De Milo, but jaw dropping mostly becuase at 43 I was so very amazed that I had actually made it to see these things.

All of the things above and the sight of the Cathedral rising above the sunset lighted plain at Salisbury :-B

What an amazing string of responses! As I read this I realized how blessed I have been to have seen many of the places referred to - but did anyone mention the temples at Abu Simbel? Maybe I missed it somewhere in there. Anyhow, truly jaw-dropping.

But come to think of it, a place that is truly inspiring and jaw-dropping is my own community of Qualicum Beach on the eastern shore of Vancouver Island. It's natural beauty is awesome.

The Cathedral in Siena Italy

The Redlight District in Amsterdam when I first saw it years ago as a college student!

1. The first time I saw the interior of Notre Dame in Paris
2. Coming out of Strasbourg Cathedral to snowfall of HUGE snowflakes (I've lived in the South for some time and don't see much snow)
3. The Grand Canyon is still amazing. I always think of the early inhabitants and explorers who just "happened" upon it.

My first trip out of the country 30 yrs ago to Greece, 1st stop Athens, getting into the hotel room, stepping out onto the balcony, looking off in the distance and seeing the Acropolis. It was magic. Later, cruising the Aegean and going up on deck alone watching a full moon shining on the water. Many other memories until two years ago, cruising through the "three Gorges" of the Yangtze and weeping with the sheer exquisite beauty.

These are not all travel-related, but what the heck:

I concur with:
David
The Colosseum
Highway 1 along the California coast

How about:
Canoeing on swimming-pool-blue Morraine Lake (near Lake Louise in Banff), looking up into a rainbow

The pedestrian promenade in Vienna, with performers singing arias in the street

The Las Vegas Strip--different kind of awe, of course

Watching a tornado form in the midwestern sky

The San Diego sky the first full day of the fires--blue, gray, purple, pink, orange, yellow, and white, with a red sun. Beautiful and frightening at the same time.

The Greek ruins in Paestum, Italy. There stood the temples out in a field with wild flowers growing out of the tops of the columns. The sky was brilliant blue with white clouds. We were the only people there and it was like stepping back in time. A truly glorious sight.

Howard, what a nice thread. A lot of emotions were evoked upon reading this.

There have been several sites that caused a jaw dropping experience...
the first time to St Peter's Basilica. It was even larger than I expected.
The Pena Palace in Sintra, Portugal, engulfed with fog making it really look mystical. Niagra Falls, larger than life, again. The Kunstehistoriche museum in Vienna...all that marble and granite!

But, far and away, it was seeing and walking on The Great Wall of China. It remains the most incredible site to me. The day was bright with sunshine, the air with a snap of cold, and no one around for hours at a time. The world lay at our feet when we got to the highest point and what a wonder to see the wall move beyond in both directions farther than my eyes would see.

Standing on a ridge outside Todos Santos in Baja on July 11, 1991. Watched the sun set all around us and then bathed in seven minutes of total eclipse.

Also: the evening sky in La Paz two days earlier had the most eerie, intense orange glare I've ever seen.

Besides those, I'll remember Venice (cruising out to the Lido and back at night), the Gornergratt vista in Zurich, the lakeside promenade in Montreux, a post-snowfall breakfast in the El Tovar at the Grand Canyon, the nothern lights seen from an Alaskan ferry, Val Poschiavo from the Bernina train, arriving at Ravello during a cloudburst and walking out into our patio there.

And a lot more....

Definitely waking up early and watching the sunrise over the Grand Canyon. Something everyone should see to appreciate.

The view from the airplane when flying into Barcelona.

Arlington National Cemetary.

Vegas, not the most beautiful site, but the sheer enormity of everything! My husband and I saw it for the first time the night before we were married there.

Oh, so many. Here are a few:
- fields of giant lupines in Prince Edward Island
- Approaching Soglio, Switzerland at dusk from a high mountain trail
- approaching the Bonhomme Refuge at dusk on the Tour de Mt Blanc
- sunset, sunrise and full moon nights at the bottom of the Grand Canyon
- Hiking to Grindewald First in the Swiss Berner Oberland
- Northern New England during color change
- biking across Logan Pass, Glacier National park, as the sun peaked out after a wild rain storm. Just gorgeous.
- Biking in the mountains of Andalusia near Arcos de la Frontera.
- Biking through the Morvan of central France
- a lightening storm seen from Harbor Island, Penobscot Bay, Maine
- Notre Dame Cathedral
- Noyers,France
- the Maine coast especially Recompense Bay, Maine
- the Mall of America - not a pleasant sight, but impressive
- Ground zero shortly afer 9/11
- I would add aromas to the list: Autumn in northern New England; riding down a road in France lined with apples trees during harvest; lupines in PEI; early moring along the Maine coast
- Saving the best for last - Seeing my wife of 32 years at the end of each workday, and riding along we me on bike adventures