"Essential" gear is a relative term. Each situation and budget has its own necessities. But this video of noted photog Chase Jarvis explaining his core kits is a solid breakdown of how to be ready for any situation.
Chase Jarvis, a guy who's a good enough photographer that Nikon asked him to advance test the Nikon D90, just put out a photo-book called "The Best Camera Is The One That's With You". He only used an iPhone.