The Political Economy of Football website has published a list of the top 30 European clubs in attendance of domestic league matches over the past three seasons. It makes for very interesting reading. The top three or four teams are those that you would expect to see, such as Manchester United, Barcelona, and Real Madrid, as well as Borussia Dortmund with their massive stadium and the best fan support in Germany. It's only in the last two seasons that Barcelona have entered the top two in attendance for domestic matches, taking the top spot in the 2010-11 season.
There are some themes that become apparent upon studying the list. First, the Spanish and Scottish leagues are very unbalanced at the top, with Valencia and Atlético Madrid being the only sides outside of the Big Two who are in the top 30 list. Second, the German Bundesliga has massive fan support up and down the league. England has a large number of clubs in the top 30, but even Bundesliga teams in the bottom of the top flight enjoy average attendance of over 40,000 per game. Third, Chelsea are a medium-sized club with extra-large ambitions — their attendance in the Premier League is barely in the top 30 in Europe, and barely more than Sunderland's! When one sees those numbers, perhaps the departure of three of Chelsea's scouting staff for Hamburg makes some sense.
I would love to see a similar chart of the top 30 European clubs' league attendance relative to stadium capacity (which is different for German Bundesliga matches than Champions League matches due to rules on standing areas).