Apparently, this laughing baby (aka William Nilsson) is so well known that he is known as THE laughing baby. He even has his own wikipedia entry. Here’s what they say:
The Laughing Baby is an Internet neologism given to a YouTube viral video of a baby laughing. The video has become an internet phenomenon with over 6 million views (The “Hahaha ” version by BlackOleg, which is exactly the same as the original, was viewed for mais de 103 million times as of Sep. 2008). Uploaded by a Swedish man with the pseudonym spacelord72, and later re-uploaded and popularized by another user known as BlackOleg,  the “Laughing Baby” is one of the few internet memes that have entered popular culture.
I wonder what else makes him laugh? (Most things, I imagine)