Gosh where to start. Lots of performances that are not in the public domain and then lots that are...
Here is one that comes to mind
http://www.youtube.com/watch?v=q9Ave90kGpQ&feature=youtube_gdata_player
...and my only concern with quoting examples is that this turns into some witch hunt where we try to discredit every significant sporting achievement presented. But trusting that this is not your motive. The above is a video of an unknown pair of brothers who defeated the 7 times world champions in an unbelievable fashion at the 1992 Olympics. Watch it it's fun - listen to the last 5 seconds of the video.
I could post things like this all night but I am not convinced that this will persuade anyone of anything. We will all believe what we choose to believe.
AND please please no one post that this is different from the TdF. I was asked to show that there are performances in sport that are difficult to believe but happen without doping and here is one.