Stefferdoos

Just 3:

US/Canada - car at multiple crossings (Alex Bay, Niagara Falls, Detroit/Windsor).

Netherlands/Belgium - car - does this even count anymore? There would have been more on this particular trip had it not been a bank holiday and I was having trouble finding a way to get my bank card to work at any atm or gas station in Belgium. So I had to turn around once I got to Antwerp and head back. Was a nice drive over the dams on the way from Rotterdam though.

Hong Kong/PRC - train/foot

And I suppose just for the uniqueness of it but not really a border. Europe/Asia via car over the Bosphorus Bridge in Istanbul

Stefferdoos

Seeing as how it still requires a passport and a visa I'd argue yes.

Oxb

On foot:

USA<>Canada
USA<>Mexico (also by automobile)
Italy <> Vatican City

By rail:
Netherlands <> Belgium
UK <> France
Germany > Switzerland
Switzerland <> Austria
Switzerland > Lichtenstein
Lichtenstein > Austria
Austria <> Germany
Switzerland > Italy

manneca

Not a lot; but lots of unusual ones:
US-Canada by foot, car and kayak (different trips)
Khazakstan-Uzbekistan
Uzbekistan-Krygyrstan (and reverse)
Uzbekistan-Tadjikistan
Uzbekistan-Turkmenistan (and reverse)
all Uzbek crossings are by foot cause that's the only way to go. Longest was 1.5 km in the hot sun
China-Hong Kong (train)
Switzerland-Leichtenstein
Italy-Switzerland
Kenya-Tanzania
Uganda-Rwanda
not exactly a border crossing but rafted Zambesi border between Zambia and Zimbawbwe
Nicaragua-Hondurus (illegally and by boat so i could buy a Coca Cola during the Contra wars; visited an army encampment on the Honduran side to purchase said soft drink)
Italy-Vatican City (foot)
Italy-San Marino
Israel-Egypt
Egypt-Jordan (ferry)
Peru-Bolivia
Did a multi country Baltic cruise, but that seems a little strange to count

Yaatri

U.S.-Canada (Car)
Portugal-Spain (Car)
Spain-France (Car)
Luxembourg-Germany (Car)
Luxembourg-France (Car)
France-Belgium (Car)
Austria-Germany (Car)
Hungary-Slovakia(Train)
Slovakia-Czech Republic (Train)
Romania-Moldova (Train and MaxiTaxi)
Turkey-Bulgaria(Bus)
Singapore-Malaysia (Bus, Train)
Malaysia-Thailand (Foot one way, ferry on return)
Thailand-Cambodia (Foot)
SIngapore-Indonesia (Ferry)

I did not snap my fingers even once.

mhnadel

Interesting question. Let's see:

1. USA - Canada and v.v. (car)
2. Belgium - Netherlands (train)
3. Netherlands - (West) Germany (train)
4. (West) Germany - Denmark and v.v. (train / ferry)
5. (West) Germany - Austria and v.v. (train) (2 different border crossing points)
6. (West) Germany - France (train)
7. France - U.K. (England) (ferry)
8. Italy - Holy See and v.v. (foot)
8. Kenya - Tanzania (overland truck)
9. Tanzania - Malawi (overland truck)
10. Malawi - Zambia (overland truck)
11. Zambia - Zimbabwe (overland truck) (also back and forth on foot several times at a different border crossing)
12. Zimbabwe - South Africa (overland truck)
13. South Africa - Botswana (overland truck)
14. Botswana - Zimbabwe (overland truck)
15. Namibia - South Africa (bus)
16. South Africa - Swaziland and vice versa (bus) (2 different border crossing points)
17. Russia - Mongolia (train)
18. Azerbaijan - Georgia (car)
19. Georgia - Armenia (car)
20. Greece - Bulgaria (train)
21. Ghana - Togo and v.v. (car)
Togo - Benin and v.v. (car)
22. Belgium - Luxembourg and vice versa (train)
23. Austria - Slovak Republic and vice versa (train, boat)

I've also traveled by sea from South Africa to The Gambia (via St. Helena and Ascension Island) and on to Spain (the Canary Islands) and the U.K. but those are long enough distances I'm not sure I'd count them. That is, the ship was not just for the border crossing.

21H21J

From what I can remember I've done the following, mostly by car and commercial truck (which was not much fun in the days when you had to do customs paperwork at every crossing)

UK <> Ireland
UK (Gibraltar) <> Spain
France <> Spain
France <> Switzerland
France <> Belguim
France <> Italy
Belguim <> Germany
Belguim <> Holland
Germany <> Switzerland
Germany <> Austria
Germany <> Czech Republic
Germany <> Holland
Austria <> Italy
Slovakia <> Hungary
USA <> Canada

Mr H

Europe - heaps between the following countries:

GB
IRL
F
E
B
NL
L
D
A
CH
I
V
RSM
DK
S
N
FIN
EST (by sea from FIN)

Asia

SGP-RI (ferry)
SGP-MAL (rail)
MAL-T (rail)
T-LAO (ferry)
LAO-VN (foot)

Africa

ZW-Z (foot)
Z-BW (road)
BW-NAM (road)
NAM-ZA (road)

grizu

One of the interesting train rides: I once went directly from Narvik (Northern Norway) to Germany via Sweden and Denmark. It is 3000km and took more than 24h.

CurlyKrakow

USA <-> Canada (RT by car, BC + ON) / (by foot, one illegal street crossing in Pt. Roberts, WA just last month )
USA <-> Mexico (RT by car, TJ)
Russia <-> Ukraine (RT by overnight train, 3x)
Haiti <-> Dominican Republic (RT by car, 2x)

pjoalfa

By sea:
Greece-Cyprus
Cyprus-Israel
Israel-Egypt

By Train:
France-UK
China-HK

By car:
US-Canada (also train)
Dominican Republic-Haiti
Netherlands-Belgium (also train)
Netherlands-Germany
Belgium-France (also train)
Germany-France
Germany-Czech Rep.
Czech Rep-Slovakia
Slovakia-Austria
Austria-Germany
Spain-France
Italy-Vatican (on foot)
Ireland-UK
France-Monaco

cpx

NY - NJ (Tunnel, Bridge, on foot and boat)

sipes23

Now that could be a fun thing to try for. So far I've only bicycled over state lines (IL <> WI and NH <> ME), but the prospect of an international bike ride sounds fun.

TULOKCICT

On foot:
US<->Mexico (TJ and Algadones)
Italy<->Vatican

By Car:
US<->Canada

By Train:
Germany->France
France<->Monaco

By Ship (technically not "border" crossings):
Italy->Croatia
Croatia->Greece
Greece-> Turkey
Turkey-> Italy
Italy->France
France->Spain

cigarhog

I've crossed US-Mex and US-Can only. Crossed many "water" borders though.