Manchester United
Manchester City (Maine Road and the new place)
Newcastle United
York City
Leeds United
Chelsea
Oxford United (Manor Ground, first game my son went to, don't know if he saw it or not as he was still a babe in arms. Oxford beat Barnsley in the latter's last match in Div 2 before they got promoted to the big boys. Believe me, it was not like watching Brazil.)
Shrewsbury (Gay Meadow)
Chelsea
Tottingham (old White Hart Lane, although I'd love to go to the new place)
Anfield
Wembley (the previous version)
Exeter City
Barnet (Underhill, against Blackpool)
Blackpool (against I can't remember)
Bury (but to watch FC United of Manchester)
Aston Villa
Nottingham Forest
Oldham Athletic
Walsall
Notts County
Derby County
Stockport County ...
and ... Newport County
Cardiff City
And a raft of non-league clubs from Bala to Stalybridge Celtic
San Siro (to my shame I can't remember who the home team was, but they won 1-0 against Verona with a 90th minute penalty and it was the only game I ever saw with my mum and the last game I saw with my dad)
Juventus
Sampdoria
Genua
Chieti
PSG
Ajax
Feyenoord
Excelsior
Sparta
MVV Maastricht
Heerenveen
ADO Den Haag
Erzurum (we started chanting along with the home fans who were giving the opposition goalkeeper some stick when someone tapped me on my shoulder and said my girlfriend -- the only female in the ground -- shouldn't be chanting because it wasn't right for a woman to be calling a man a "homo") 1-0 to the home team again, yet another 90th minute penalty.
Quite a few Dutch non-league teams
and the best of all:
FC Utrecht
Edit:
Late addition: Sunderland, the old Roker Park
Sorry even later additions:
Viktoria Plzen (on the other side of the river to the Plzen brewery, that was a day well spent!)
Zenit St Petersburg
Oh, and a tour of Barcelona's stadium. That doesn't really count, does it?
The most interesting thing in the museum was Indurain's yellow jersey! YMMV