420 is marijuana
Was going to put inn the other names - but way too many
But of course, here I go with my links - a pretty cool link I hope:
http://www.phish.net/faq/420origin.html, but as you can see, it is not true:
http://www.snopes.com/language/stories/420.htm
Ah shucks, I found:
http://www.concept420.com/what-is-420.htm
http://www.420.com/420site/lounge/index.php?page=420