Nationwide Protest of PROP 8 - NOVEMBER 15th 10:30am PST / 1:30pm EST
The site has details on protests being organised in various locales across the States. Accessing it is a sporadic affair, though -- looks like it's being linked all over the place and can't handle that many requests. If it doesn't work for you and you still want to go, try the site a little later in the week, maybe. :)