Love this! I'll probably forget some but I'll do my best.
Belgium:
Cristal Arena - home of KRC Genk: last season champions! Courtois (Atletico Madrid now) and De Bruyne (Chelsea transfer this window) are the famoust key players of that team. We played CL this year (Chelsea, Valencia and Leverkusen group). I live in Genk, so I'm in the stadium quite a lot each season!
Sclessin - home of Standard de Liege: probably known very good here, Juve and Standard worked together. Witsel, Defour and Felliani are ex-Standard players. Been there for the match Standard-Juventus, a friendly in the mid season, a few seasons ago.
Heysel - Known because of the Heysel disaster (Juve-Liverpool final). Been there for the cup final of KRC Genk against KV Mechelen, and Italy-Mexico, friendly before the WC2010
Germany
RheinEnergieStadion - home of FC Köln, Lukas Podolski. Been there for the game FC Köln - VfB Stuttgart. Lovely stadium, Germany is very lucky with the world cup in 2006, all the stadiums there are perfect!
Signal Iduna Park - home of Borussia Dortmund. Been there for 2 friendly games: Germany - Italy 1-1, and Dortmund - Juventus, 2 or 3 seasons ago. Wonderful "sudtribune", like 25.000 standing behind the goal.
BayArena - home of Leverkusen. Been there for Leverkusen - KRC Genk. Once again, a beautiful stadium!
Mercedes Benz Arena - home of VfB Stuttgart. Been there for the CL game VfB - FC Barcelona. Beautiful stadium. Was like 3 or 4 years ago.
Allianz Arena - home of FC Bayern München. Been there for the CL game against Juve! Too bad 0-0, would have been amazing if we won there

, also the Allianz Arena is a really beautiful stadium. Especially the lights at the outside: wow!
Spain
Camp Nou - Stadium tour, it's a very old but impressive stadium
Olympic Stadium Barcelona Been there, saw it, ugly.
France:
Stade de France: stadium tour, the home of the NT of France.
Parc de Princes:: stadium visit, a quick look. Home of PSG.
Italy
Juventus Stadium The most beautiful I ever saw. Wow. We have an amazing stadium. Juve-Novara, and sunday Juve-Siena for my second game. Probably next year a season ticket!
Picture album of Juventus Novara @
http://s973.photobucket.com/albums/ae214/Ledje7/Juventus Novara/
On my 'to visit' list: Stamford Bridge, Old Trafford, Wembley, Anfield, San Siro, Real Madrid, Arsenal FC, Olimpique Marseille..