The McGurk effect
While idly browsing through Wikipedia, I came across something very cool: the McGurk effect. Just read the Wikipedia page, and try it out with the linked YouTube video. I found the effect rather weird to experience :) (You can watch YouTube without creepy Flash plugins using ffplay/mplayer and the Konqueror YouTube servicemenu. The HTML for the YouTube site apparently got changed recently, so you'll have to manually patch the servicemenu script according to the last comment in that page).
Cool, I didn't know about that. But I think it's rather nice to have the servicemenu nonetheless for regular usage, since it seems to offer a slightly better workflow: you don't have to switch from the GUI to the console just to watch a video.