Right - here goes:
On foot
France / Belgium
Belgium / Netherlands
Netherlands / Germany
Belgium / Germany
Luxemburg / Germany
France / Germany
France / Switzerland
Germany / Switzerland
Switzerland / Italy
Germany / Czechia
Germany / Poland
Czechia / Poland
On foot oddities
Netherlands / Belgium at Baarle-Hertog / Baarle-Nassau (the greatest example of border lunacy in Europe, if not the world)
Netherlands / Germany at Kaltenkirchen / Venlo by falling off my bike from one into the other!
To be really pedantic, as embassies are sovereign territory: UK / Germany!
As there is a small part of the esplanade of Edinburgh Castle which was gifted to Nova Scotia can I count UK / Canada?
By train
France / Belgium
France / Germany
France / Switzerland
Switzerland / Italy
Switzerland / Liechtenstein
Switzerland / Austria
Belgium / Luxembourg
Belgium / Netherlands
Belgium / Germany
Germany / Switzerland
Germany / Austria
Germany / Czechia
Germany / Poland
Germany / Denmark (my first ever passport stamp - although that was on a train ferry!)
Denmark / Sweden (on train ferry if that counts)
Sweden / Norway
Austria / Italy
Austria / Slovakia
Austria / Hungary
Austria / Czechia
Austria / Slovenia
Czechia / Poland
Czechia / Slovakia
Slovakia / Hungary
Hungary / Romania
Hungary / Serbia
Slovenia / Croatia
Croatia / Serbia
Serbia / Montenegro
UK / Ireland
West Germany / East Germany
train oddities
the lines from Cheb (CZ) - Bad Brambach (D) and Zittau (D) to Görlitz (D) criss-cross the D / CZ or D / PL borders several times in quick succession
By Road
France / Belgium
Belgium / Netherlands
Netherlands / Germany
Belgium / Germany
Czechia / Poland
I excluded UK to France by train from mine (and presumably others have done the same). As Øresund is part bridge, part tunnel maybe it also shouldn't count? Although, I'd certainly like to cross the Sweden/Denmark border using it!
The difficulty of not counting bridges is the fact that many "land borders" are in fact delineated by watercourses - even if you can step across them in places and if travelling by train not notice that they're there at all. I also didn't count the Channel Tunnel as you can't really tell, so it feels like cheating!