I often have a hard time explaining to people what a SharePoint Saturday is... I usually use the whole "one day conference" line, but it doesn't really adequately describe the opportunity there for anyone to take advantage of. Mark Miller (of EnduserSharePoint.com fame), however, has got it right:

SharePoint Saturday is the chance to be part of a community. It’s a chance to contribute to the community. It’s a chance to be part of something larger and more emotional than an online forum or blog.

It is not a closed community. Anyone can join in and participate just by showing up.

Mark Miller