Los Angeles Department of Cultural Affairs Annual Festivals Guide, which is available online as a PDF or can be picked up at certain official city offices. It lists cultural festivals, free concerts in the parks, street fairs and so forth. I found something on pretty much every page that I'd like to do.
Another good resource, and one of that's actually online and updated regularly, is ExperienceLA.com, which is a local events calendar cosponsored by the Community Redevelopment Agency of L.A., the L.A. County Arts Commission, Metro, the L.A. Convention and Visitors' Bureau, USC, the DWP and the cities of Los Angeles, Long Beach and Santa Monica. It's searchable, and it covers an even wider variety of activities than does the festivals guide.