A tough one - loads of border crossings in Europe, mostly by car.
Germany <-> Switzerland
Germany <-> Austria
Germany <-> Belgium
Germany <-> Luxembourg
Germany <-> France
Germany <-> Czech Republic
Germany <-> Netherlands
Austria <-> Liechtenstein
Austria <-> Slovenia
Slovenia <-> Croatia
Switzerland <-> Liechtenstein
Switzerland <-> France
Switzerland <-> Italy
France <-> Luxembourg
France <-> Spain
France <-> Monaco
France <-> Italy
Belgium <-> Netherlands
Belgium <-> Luxembourg
Spain <-> Gibraltar
Probably some more that I can't remember right now.
Elsewhere in the world, I'm a bit more sure:
USA <-> Canada
Mexico -> Guatemala -> Honduras
Belize -> Mexico
Spain (Ceuta) <-> Morocco
Singapore <-> Malaysia
Thailand -> Cambodia
Lebanon <-> Syria