Craigslist is amazing for its community, though the search for rideshares uses bulletin-board style instead of a proper sort. Both early testers have found their best results for city-to-city carpooling on Craigslist; it seems especially useful up and down I5 on the West Coast. Plus Craig is our hero. It's also useful as a backup site for commutes and events if your city or event hasn't organized anything better. You have to be a bit persistent to maximize searches: try all likely spellings both when creating a post or searching, consider nearby towns or county names (to go to Arcata, also search Eureka, Humboldt, and Humbolt), and for one-way trips consider searching the Craigslist site for both your departure and arrival towns.