the majority of TdF winners over the last 20 years has some sort of doping incident on their resume.
Indurain (91-95) was popped for an agent that was prescribed for asthma users.
Bjarne Riis ('96) was nicknamed "Mr. 60%" because of the maximum allowable red-blood cell limit before being considered positive. Years later, he eventually said that he injected EPO to himself. He was removed from the TdF records for several years before being reinstated into the records in 2008 with an additional footnote.
Jan Ulrich had everything from the IOC trying to take away his gold medal, $35,000 worth of doping agents in his name, and being popped for amphethamines, before being busted this past February for a number of EPO-related things.
Marco Panati ('98) had a history of EPO and drug usage in general and also popped near 60% (see: Bjarne Riis) during his TdF days.
I think We all know Floyd Landis' story from 2006, but Óscar Pereiro (who was eventually awarded the 2006 title) popped for salbutamol, the same drug in which Indurain was popped for in '94. Both Pereiro and Indurain did have their doping rulings overturned, but is it really that hard for somebody like that to obtain a prescription?
Alberto Contador won in '07 and '09, but has a doping trail to his name from 2006 to 2010 and is currently under suspension due to popping during the 2010 TdF (which the title was eventually stripped from him.)
Once you get past 2008, you no longer have the cloud of doping that followed the sport like it had for the last 2 decades. Carlos Sastre ('08), Andy Schleck ('10, although more of a de facto winner based on Contador's doping), and Cadel Evens have been clean....so far. Based on the pattern of TdF winners, both before and after Lance, I have a hard time believing that Lance was clean. I want to believe he's clean, but the more claims that surface, the more it makes you wonder how likely that a dirty test is gonna surface.