I like trams, so quite a lot of systems
Not counting museum lines:
Benelux:
Amsterdam, Utrecht, Brussels, Antwerp, Kusttram, Luxembourg
France:
Paris, Strasbourg
UK:
London, Manchester, Blackpool, Edinburgh
Switzerland, Austria:
Basel, St. Gallen, Vienna, Innsbruck
Czech Republic, Slovakia, Hungary, Poland:
Prague, Bratislava, Budapest, Warsaw, Szczecin
Bulgaria, Romania:
Sofia, Bucharest
Latvia, Estonia:
Riga, Tallinn
Denmark, Sweden, Finland:
Aarhus, Helsinki, Stockholm
Germany:
Rostock, Schwerin, Berlin, Schöneiche-Rüdersdorf, Woltersdorf, Potsdam, Brandenburg (Havel), Frankfurt (Oder), Bremen, Hanover, Dresden, Leipzig, Chemnitz, Erfurt, Nordhausen, Bielefeld, Dortmund, Bochum-Gelsenkirchen, Essen, Mülheim-Oberhausen, Duisburg, Düsseldorf, Krefeld, Köln, Bonn, Kassel, Frankfurt (Main), Mannheim-Ludwigshafen-Heidelberg, Karlsruhe, Stuttgart, Freiburg, Saarbrücken, Würzburg, Nuremberg, Munich