US - Mexico. Parked at a McDonalds just north of the border and walked to Tijuana. Only spent about four hours there, in daylight, or else I wouldn’t have done that.
US - Canada. I was a passenger in a car at the border station south of Vancouver, and we were sitting in line waiting to cross into the US. We were a few feet before the stone markers showing the actual border, so I jumped out of the car and walked across and back just for grins.
CA - NV. Gambling in a hotel in Stateline, NV. Went to get some fresh air and strolled across the street into California and back.
KY - OH. Stayed in a hotel in Covington and walked over the bridge to see a Reds game.
VA - TN. The town of Bristol straddles both states. Stayed on the Tennessee side and walked about a mile to the Minor League Ballpark in Virginia. Afterward we hung out on State Street, which is the state line. All the bars are in Tennessee, though.
Uzbekistan -> Turkmenistan (Got a car to drop us at the border only to find that the crossing didn’t open for another hour or two, so had to hang around a while. Despite horror-stories of bribes and people getting all sorts of things confiscated because of minor discrepancies with customs declarations, the border guards were generally great: “No drugs? No guns? No…nuclear?!”)
Oman -> U.A.E. (had taken a bus back to UAE that didn’t take us through border control, so had to get someone to drop us off around the other side of a border post in the middle of the desert, then drive around and pick us up from the other side - looked very strange walking through the drive-through only border post)
Cambodia -> Thailand (Dropped off by tour bus on one side, met different guide on other side for the rest of the tour. Metres from the border and nobody would change our leftover Cambodian cash)
The mailing address was SD (Big Stone City) on a rural route. Since the mailbox was in MN, wen my mother wanted to move it to the SD side of the road, she wrote a letter to Senator McGovern - he got it moved! (It got moved back a few years later for efficiency sake).
Despite being a “border enthusiast”, and crossed several (a paltry number when compared with most people, enthusiast or not, but pretty decent if I say so considering my limited resources to travel), I’ve only crossed two on foot that I can recall: Manitoba-Minnesota many years back when I wanted to ride my bike to the US. Started at the park my family was camped in, took 3 hours there (got there for lunch, sat down right at the cairn marking the line to eat, I was in Canada, my sandwich was in the US, turned around at the duty free shop in MN, and rode back.
The second one was when I went to visit my sister. She lives in Emerson, MB, her house is just 300 feet from the border (I measured). One day when we went to visit, I walked from her place to the point where Manitoba, Minnesota, and North Dakota meet, and took some pictures there. They can be seen here, on Flickr] (as well as the two pictures previous). This was taken in the winter for obvious reasons.