The Cancellara video looks a little odd just because of the dynamics of bike racing. If you interpret the video as Cancellara pulling away from other pros like that with everyone going full effort, it looks super human. In reality though, Cancellara pulls away from the group so quickly at 3:35 (the time you linked) because no one is trying to go with him at that moment.
He's attacking with ~50km left, which is far enough from the finish at a major one-day race (Paris-Roubaix here) that the chances of the main group reeling him back in are pretty high (he did go on to win that day). As another rider in that group, you have two options when you see someone attack: A) try to go with Cancellara's move and hope that you can beat him one on one (unlikely, Cancellara is one of the all time greats and 2010 was a year of peak form for him), or B) stay with the pack and hope that you can collectively haul him back later in the race (now you've saved your own energy and Cancellara has spent some of his).
Look at the lead rider in the pack (white jersey, blue shorts) at 3:46 as Cancellara goes past him. You can see him look back at the other riders in the group. He's checking to see if anyone else is deciding to do with Cancellara. No one else moves, so they maintain a steady pace, hoping to bring him back with option B.
In the other video, Hesjedal's bike spins because of the road gradient and momentum in his still spinning wheel hitting the road as he stands up. Hard to see the slope of the road from the camera angle but they are on a steep descent.
Mechanical doping is definitely a technological possibility and the UCI is starting to test bikes. That said, the "bike fans" who point to these videos out as evidence are conspiracy theorists. The explanations I wrote above are IMO pretty apparent to someone who has raced bikes before, but I can see how they look odd to a layman observer or a non-competitive cyclist (in the case of the Cancellara video especially).
Edit: the race I was referring to was actually Paris-Roubaix, not Flanders. This article has some pretty good post-race interview commentary from Boonen and Cancellara himself about how the effort to chase him down sputtered because of a lack of cooperation, not ability: http://www.telegraph.co.uk/sport/othersports/cycling/7578607...