As we get ready for the season finale tonight, it seems like everybody has a theory about who or what Jacob is and what his purpose on the island is. Hopefully we find out tonight, but in the meantime, feel free to critique my theory about the mysterious Others' leader.
I think Jacob is John Locke. Locke will be ever so surprised when he goes to kill Jacob to find out that it is a "future" version of himself. I have future in quotes because, as this season has made quite apparent, the future is relative. Suffice to say that Locke hasn't become Jacob yet, but perhaps he will become Jacob in tonight's episode if he succeeds in killing him.
So here's how it will probably go down. Locke kills Jacob and then discovers that Jacob is his future self. Locke then assumes the role of Jacob and takes over leadership of the Others. Using the wheel or some other means of time travel, Locke travels back and forth throughout the island's history to fulfill Jacob's duties.
Remember when Ben first took Locke to see Jacob he said, "This isn't a man you go to visit. This is a man who summons you." If Locke does become Jacob and uses time travel, then he obviously couldn't be in the cabin twenty-four hours a day. So perhaps that's why Jacob/Locke summons people, because he's not there all the time. Locke is the only one who has gone to Jacob without being summoned, but if Jacob really is a future version of Locke then he would know exactly when Locke would arrive at the cabin.
Anyway, after a long life of being Jacob, Locke will eventually make the ultimate sacrifice, returning to 2008 so that he can die at the hands of his younger self. There are a few holes in the theory. I can't explain why Jacob hates technology or where he lived before Horace built the cabin, but overall I think it's a pretty solid theory. Let me know what you all think.