DropBox Alternatives (For Linux)

Of late, I've been looking for alternatives to DropBox for automatic file syncing across multiple computers; for syncing between my laptops and my desktops (yes, I am a geek: I have four computers(*))
I didn't even bother trying out DropBox, because I don't want to put my data in the hands of a third party, not when I could run a server myself. It's not even primarily whether I can trust the third party, though that is a factor. It's that third parties tend to vanish, and there goes one's data.
( so what are these alternatives? )