The Roald Amundsen Monument outside Golden Gate Park, SF. Amundsen had no connection to SF, and I don’t think he ever even passed thru, but one of his boats ended up in SF, at least for a while.
I hope he didn’t ask the locals for the way to Carnegie Hall. 
I’m sure others have seen this, but I noticed it several years ago when we were walking to Oracle Park to watch a Giants game.
If you can stretch “visited” to “drove past and googled later”, a memorial to the Long Cane Massacre.
Ran across this with text while walking between the train station and the train museum in Utrecht. Wilhelm Röntgen was expelled from Utrecht college.
On the tropical isle of Langkawi, the legend of Mahsuri lives on.
I visited in 1996. There was a sign next to this sign with a picture of a young girl of the seventh generation. Right around the time she was born, tourism in Langkawi picked up and prosperity returned to the island as in the prophecy.
Right near the laundry place that I used to patronize before I moved into an apartment that has a washer and dryer is a marker devoted to Buck v. Bell, one of the more shameful decisions our hallowed Supreme Court has ever rendered. Carrie Buck, whose involuntary sterilization was made possible thanks to the decision, was born here in 1906.
Heh. I used to live on that block in Medford, MA where that marker sits.
I’ve got a few – In the section of West Windsor, NJ that was known as “Grovers Mill” there’s a monument to the infamous Orson Welles 1938 Halloween broadcast of H.G. Wells’ War of the Worlds
https://www.atlasobscura.com/places/monument-to-the-war-of-the-worlds
My wife grew up in Grovers Mill, and her sister was instrumental in getting the monument erected.
What’s really interesting is that nothing actually happened there, unless you count the (exaggerated or possibly fictional) accounts of people shooting at the oddly-shaped water tower at the Grovers Mill company, thinking it was a Martian. (The water tower, and the Grovers Mill company are both still there)
Frankenstein Trestle marker near Crawford Notch in New Hampshire. The railroad trestle was near Frankenstein Bluff in the White mountains, which had been painted by Godfrey Frankenstein, a member of a family of German artists who migrated to the US about the time Mary Shelley’s book came out, and were unaware that a famous mad scientist had been grafted onto the family tree. One of the arliest movies was about a train going over the FRamnkenstein Trestle, which makes it the first movie with “Frankenstein” in the title.
https://www.hmdb.org/m.asp?m=77698
There’s a marker by the roadside in Westford Massachusetts to Prince Henry Sinclair of the Orkney Islands, who some claim voyaged to America a century before Columbus and left a rock carving of a knight, possibly a grave marker for a companion who died there:
https://www.hmdb.org/m.asp?m=139656
https://www.hmdb.org/m.asp?m=230425
http://www.clansinclairsc.org/images/Prince%20Henry%20Sinclair%20Brochure.pdf
My wife Pepper Mill just pointed this one out to me yesterday. I haven’t visited it yet, but it’s worth the trip.
There’s a “blue marker” in Linlithgow, Scotland, which is where Montgomery Scott will be born on June 28 2222 . This site even has a countdown clock:
Take that, James T. Kirk!
Your bit about Grover’s Mill reminded me of this fun thing I’ve visited.
It isn’t quite a historic marker. But it is historic and it marks a spot.
At the risk of running the digression too far, if you’re talking about huge containers that boast about their holding capacity, you have to mention Boston’s Giant Tea Pot. It dates from 1873, when it was erected outside the Oriental Tea Company. The giant tea pot emitted clouds of smoke, and is marked with its capacity – 227 gallons, two quarts, one pint, and three gills.
The pot has remained there through multiple ownerships, still putting out smoke. Now it’s attached to a Starbucks.
To bring this back on track to Historical Markers you’ve Visited, I submit the Betty and Barnet Hill Alien Abduction marker near Lincoln, NH on Route 3. This was the couple who claimed they were abducted in 1961, and which formed the basis for John Fuller’s book The Interrupted Journey. I’m kinda surprised about the spot – I’d always pictured some out-of-the-way dirt road, but Route 3 is a pretty imortant north-south route, although INterstate I-93 has taken a lot of the traffic off it. Still, I drive on this frequemntly enough, and have a hard time picturing an abduction there.
https://www.hmdb.org/m.asp?m=74571
https://www.tripadvisor.com/Attraction_Review-g46140-d8612098-Reviews-Betty_and_Barney_Hill_Incident_Historical_Marker-Lincoln_New_Hampshire.html
Please – no comments. I’m a skeptic myself, and know the doctor who examined them didn’t believe an alien abduction took place.
This one is located not far from where I lived in Salt Lake City Utah.
It’s a monument to a tree stump.
Really.
There was apparently only one tree growing in the Salt Lake Valley when the Mormon pioneers came – or at least near their track from Emigration Canyon to the future downtown. All the later emigrants, including the “handcart pioneers” passed close by that cedar tree, and it became a familiar and favored spot.
The tree was fenced in in 1924 to protect it. The Daughters of the Utah pioneers erected the monument to the Lone Cedar Tree in 1933.
In 1958 vandals cut away most of the remaining tree, leaving only a stump. A bronze replica was installed.
Some bits of the original wood were still there when I lived there, but it’s all gone now.
This historic marker / statue is near me:
As detailed here, there were / are more commemorations to these intrepid dudes along the route, but this is the best known. I’ve visited two of them.
The most interesting one for me is the Nevada nuclear test marker (My favourite factoid about living in Nevada is I was living in the state where IIRC 30% of the atomic bombs ever detonated by the human race exploded 
)…
https://www.hmdb.org/m.asp?m=32982
Also the Rufus Stone, the site of the “accidental” death of King William Rufus
I’ve long thought that you could do an entire book on markers and monuments to pre-Columbian “discoverers of America”. I’ve already listed Prince henry Sinclair above.
There’s a statue of Leif Ericon on Charlesbank WEst in the median of Commonwealth Avenue in Back Bay Boston. If you look closely, you’ll note that the statue’s plinth forms a rather stubby Viking longship:
Another marker is at Mystery Hill in North Salem, New Hampshire, now called America’s Stonehenge because of the supposed alignment of some of the monumental stones, in the same way such alignments exist at the original Stonehenge on Salisbury Plain in the UK. They are so committed to this that they’ve actually cut “avenues” through the trees surrounding the hill so that devoted watchers could actually see the sunrises and sunsets at solstices. (unfortunately for the owners of Mystery Hill, it’s not on a plain, so cutting don the intervening trees was necessary).
Officially, the stone constructions are the work of the Pattee family that lived there, but enthusiasts have attributed them to a host of other visitors – Vikings, Irish monks, an Icelandic bishop, Phoenicians. Even the local Indians, who would seem to be the likelest candidates. Part of the problem is that some of the reconstructors felt onbliged to rebuild the structures the way they thought they were originally.
It’s evben been argued that the site inspired the “Devil’s Hopyard” in the H.P. Lovecraft story “The Dunwich Horror”. Lovecraft scholar J.T. Joshi says that can’t be – Lovecraft didn’t actually visit the site until after he wrote the story. But her certainly could have read about it.
Anyway, I recommend it as a fun day out and a walk in the New Hampshire woods.
I’ve seen the geographical marker near Poniatowski, Wisconsin. 45×90 points - Wikipedia
In Menahga, MN: a statue honoring St Urho
