You can see the borders of North Korea with China and South Korea at night from the ISS since unlike their neighbours, they have almost no light. That got me thinking, which other country borders (of directly adjacent countries with no large bodies of water in between) are visible from space?
The border between India and Pakistan is one of the most heavily guarded and well-lit borders in the world, so much so that it can be seen from space at night. It covers an immense distance from the Arabian Sea to the Himalayan foothills.
Here is how it looks from ISS. India-Pakistan Border at Night from NASA Earth Observatory
The winding border between Pakistan and India is lit by security lights that have a distinct orange tone. The port city of Karachi is the bright cluster of lights facing the Arabian Sea. For scale, the distance from Karachi to the foothills of the Himalaya Mountains is 1,160 kilometers (720 miles)
Another interesting historical divide that can be seen even today is the effect of the Berlin wall. Despite the reunification, the remnants can be still seen today. Berlin at Night by ESA astronaut André Kuipers. A zoomed-in version of the original image from the Washington Post is shown below.
The former division between East and West Berlin can be seen. The yellow lights correspond to East Berlin and the greener tones show West Berlin. The difference in colour is due to yellow/orange sodium lamps in erstwhile East Berlin and white-green mercury lamps in West Berlin
Thanks to @winny for pointing out the reason for the difference in the hues.
You did not specify if you are thinking of any border or only an anthropogenic one.
Anthropogenic (they will likely be visible due to different levels of economic development):
Egypt and Israel (different level of development visible on both sides)
Haiti and Dominican Republic (less vegetation in Haiti)
Natural ones (rivers and mountain ranges can be seen from space):
Mexico and USA (Rio Grande)
Germany and France (Rhine)
Germany and Poland (Oder)
While LED lighting is taking over (and is likely CW), plenty of outdoor street lights in cities and highways still use high pressure mercury and sodium discharge lamps running on mains AC voltage. They don't rectify and so will produce two pulses per cycle of AC.
Therefore any technique that can chop up time, a rolling shutter effect or simple aliasing in time from a video device, or even a spinning piece of paper with slits on the end of a space-rated paperclip would be able to distinguish a 50Hz country from a 60Hz country.
So Suriname|French Guyana or Argentena+Chile+Uraguay+Paraguay+Boliva|north thereof or Saudi Arabia|Neighbors or perhaps Liberia|Neighbors (needs better data)
For even more fun, just have a look within Japan, Tokyo prefecture and north is 50 Hz, and south of it is 60 Hz!
Two screen shots from the YouTube video 50 Hz vs 60 Hz vs 400 hz A.C. Hum Sound Comparsion converted into GIF:
click/open separately to view full size
Most of the answers were concerning night time pictures. Let me add some nice example of day time pictures, of Israel.
Plenty of grazing animals on the Egyptian side of the border, but none on the Israeli side. Gaza is distinct from both, evident by the densely populated areas and small subdivisions of farms. Compare to the larger farms in Israel, and no farms in Egypt (except along the coast).
The Israeli Golan Heights are pretty much empty and most of the area is grassland. On the other hand, the Syrian side of the border has many small villages and roads, and grassland is affected by grazing.
The Yatir Forest in Israel is an artificial planted forest. Naturally, it stops at the border.
A part of the border between Austria, Switzerland and Germany may be seen from orbit.
For the biggest part of the Lake Constance, the Upper Lake, the location of the borders within the water is not defined. Therefore the lake itself is the border, a very wide one, easily to be seen from orbit at day and night.
Within a smaller part, the Lower Lake, the border within the water is defined.
Another part of the border between France and Germany is the river Rhein between the towns Basel and Karlsruhe, see this chart.
Belgium has road lighting on almost all of its roads. The neighboring countries don't. The southern Netherlands, the Ruhrgebiet and the Lille area are densely populated so the distinction is less clear here.
Looking at this high resolution image of the Earth at night, here are a few borders I could find.
India/ Pakistan is VERY noticeable.
North/ South Korea is also VERY obvious.
Italy seems pretty obvious, but it is probably because the northern end of the country isn't very habitable.