Just a chance to meet people from the site. We hire a casino company to cater the event, give away "chips" to play blackjack, poker, roulette, etc. for a couple hours and give out prizes.
The idea started when PASS offered us a fee (US$50) for each person we could refer to the conference. They wanted exposure to build the conference and we were happy to help. At the time, Andy, Brian, and I were running the site and didn't want to make money off this (felt a little funny, ethically), so we decided to throw a party to spend all the money we got from PASS. It's a pretty good sum ($5-10k), so we have a good party.
The highlight of my year is usually going to Best Buy and spending $2k on games, DVDs, iPods, etc. to give away. It's usually a good set of prizes (40-60) for 150 people.
I'll try to take more pictures and video this year.