Quebecor/Sun Media and Bell are big examples, at least in Quebec. As of 2014 NHL playoffs, everytime the Montreal Canadiens play a game, the TVA Sports/JdQ/Canoe websites slows down to a crawl (about 1 minute loading, if it loads at all), especially near the playoffs. I hope that they fixed it for the 2014-15 season because they got the NHL rights for 12 years now, meaning even more traffic. RDS website (the French version of TSN) has similar problems, but mostly with their video section, which goes unreachable until hours after the hockey game ended unless it's early in the season.
The Videotron (Cable TV/ISP) used to be as bad as the rest of the Canoe/TVA website network but this was fixed around 2011 I think. It was kinda ironic that an ISP's official website was that hard to reach, when it was actually reachable at all. However, half of the automated customer service sections are closed outside those hours for maintenance:
Imagine if Google Hangout was closed from 11 PM to 7 PM everynight for maintenance.
Reverbnation is another example, although it has about 1% downtime. It's just that the site is atrociously slow half of the time.