Sutro Project / Pink Saturday
"San Francisco's largest underground street party" according to Wikipedia. With 500,000 attendees, it's unclear how much bigger it would be as an official event :)
Also referred to as the "Pink Party" - it's a huge party on a huge weekend. The Dyke March coincides but finishes in the Castro enabling commitment-phobes to enjoy both.
Wikipedia: http://en.wikipedia.org/wiki/Pink_Saturday
