Recently, with a group of friends, we'd be talking and then we'd hear a MacBook's fan spin-up. After questioning we'd find it's due to a flash advert on a webpage. Flash appears to be one of the worst made plug-ins ever.
So what's the fix?
In some cases it's possible to disable flash or use Adblock, but there's one glaring place where you can't: Youtube.
The solution I've found is a bookmarklet, which replaces the Flash player in Youtube with a Quicktime plugin. Quicktime's CPU usage is much lower than Flash's therefore leading to less need for the fan to speed up.
First of all you need to go to Joey Hagedorn's blogpost about it, then follow the instructions to bookmark it. I have it saved as the third item in my Bookmarks Bar. Therefore, everytime I go to Youtube, i hit Cmd + 3 and the Flash player is replaced with a Quicktime plugin. Sometimes I've found that the Quicktime plugin doesn't work, but this might be due to Youtube than the plugin itself.
Either way, for all those times you're watching Youtube videos and don't want the whirring of you fan, just switch it to a Quicktime plugin.