Most people aren't certain about the origin of Butter Lap, but the most credible history is that it began as a cyclocross training ride sometime in 2007. Since then, it has become a weekly institution - a 15ish-mile ride, gaining around 1,200 feet along the way, passing through some of the most scenic spots in the city: the GGB overlooks and the Legion of Honor. It ends with a no-pedaling "coaster race" down Point Lobos, right past the Cliff House.
Everyone who does it seems to have a lot of fun. The pace is spirited, but there are several stopping points for regrouping and conversation and sharing snacks. The ride is about 15 miles and officially ends at the Panhandle, usually between 9-9:30pm. Every ride at least some riders continue on to Bender's in the Mission for post-ride libations. Bender's is a bike messenger bar with indoor bike parking, in case you're paranoid about locking your bike outdoors at 19th and SVN.
Here's a link to a route map and specs, just to the Panhandle: http://connect.garmin.com/activity/179278227.
Here's a link to a route map and specs, with the Bender's extension: http://connect.garmin.com/activity/179278227.
This ride is NOT Urban Trail organized, but don't fret if there are no RSVP's. I've never been on a Butter Lap ride where no one attending knew the route or was ready to lead. The group can range from 5 - especially if it's raining, or if the Giants are in the playoffs - to 50 - if it's a t-shirt, Indian-summer evening.
Don't forget your lights!