Hands-down, it's clearly the best general-purpose search engine on the Web. Craigslist was set up to be primarily a local resource, and there are so many places that have broader scopes that they don't try to appeal to them. Wired's How-To Wiki details how to flex your Google-fu to get Craigslist results from every Craigslist domain.
How To Search All Craigslist at Once | Ubergizmo (Hulda Craig)
Search Craigslist Nationwide Using Google Wallpaper