Department of "That makes sense"

From HowStuffWorks.com, who makes my favorite podcast (besides This American Life), Stuff You Should Know

They had one recently on "How Deja Vu Works" - this is the theory I liked the best and found most plausible:

The Hologram Theory
Dutch psychiatrist Hermon Sno proposed the idea that memories are like holograms, meaning that you can recreate the entire three-dimensional image from any fragment of the whole. The smaller the fragment, however, the fuzzier the ultimate picture. Déjà vu, he says, happens when some detail in the environment we are currently in (a sight, sound, smell, et cetera) is similar to some remnant of a memory of our past and our brain recreates an entire scene from that fragment.

Other researchers also agree that some small piece of familiarity may be the seed that creates the déjà vu feeling. For example, you might go for a ride with a friend in an old 1964 Plymouth and have a strong déjà vu experience without actually remembering (or even being aware of the fact) that your grandfather had the same type of car and you're actually remembering riding in that car as a small child. Things like the smell and the look and feel of the seat or dashboard can bring back memories you didn't even know you had.

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"Also there is something called Ja me Vu (I think that's how it is spelled). It's the opposite of Deja Vu. In Jame Vu, you are in a familiar place, but you feel like you've never been there before. A type of alienation from your surroundings." (source)