An activity common at LEGO club meetings, in which LEGO fans each bring a copy of a LEGO set, sort out all of the LEGO elements, and take turns picking the parts they want. An easy way to get the parts you want in larger quantities without buying multiple copies of the set yourself. Read more on SEALUG.org
A "mega-draft" is basically the same as a "draft", just with a much more expensive set, usually around $100 bucks, and we typically limit the number of participants to around 6-8 to ensure that everyone gets their money's worth.
At SEALUG, we also typically use a true "snake" picking system, that is, the picks would go: 1-2-3-4-5-5-4-3-2-1-1-2-3...., such that picker #5 and #1 end up going twice in a row. This helps it be more fair to the guy who gets stuck picking last.
I hope that helps!