YouTube (youtube.com) is a video-sharing website that lets people create "channels" and upload videos for others to rate and discuss.
SuperMarioGlitchy4, who publishes his videos on YouTube, has shown and referenced YouTube several times. For instance, Account Loss had somebody hacking into SMG4's YouTube account, SMG4 VS SMG3 involves SuperMarioGlitchy3 copying SMG4's videos, Breaking Walls consisted of Mario and SMG4 breaking out of their own video and messing with the latter's account, and so on.
Later bloopers also cameo YouTube whenever the plot calls for a video. Such a blooper is The Idea Block, which revolved around SMG4's videos but only actually showed YouTube in an excerpt of a terrible video Bowser made.