“Kony 2012” created by nonprofit organization Invisible Children is the most famous viral video so far (as of March 2012).

Kony 2012 is a documentary film about Ugandan rebel leader Joseph Kony. The 30 minutes long film was viewed more than 100 million times in less than a week. Popular videos like Rebecca Black’s “Friday, Justin Bieber’s “Baby” and “Charlie Bit My Finger” took a lot longer to surpass 100 million views.

According the msn.com the most viewed video of all time (as of 2011, anyway) was Star Wars Kid - a boy acting out Star Wars scenes in his room. There are several top ten lists, which you can find if you type 'most viewed viral videos' in a search engine. It's only fair to warn you that some of the most viewed videos have adult content.

Most viewed may not exactly mean most famous, of course, and it's harder to say which clip is the most known. For example, the famous clip of Susan Boyle's first appearance on Britain's Got Talent has about 90 million hits, but it's known to many more people who have never actually watched it.

As more and more people get connected to the internet around the world, you can expect the viewing figures for viral videos to keep rising until internet use reaches its peak - which is still some way off. This won't mean that the new videos are necessarily more interesting or enjoyable than others, just that the audience is larger.