MagicFolder
Say you regularly use more than one computer. Home and office, heavy duty desktop and light portable, doesn't matter. You probably want some measure of synchronization between them, like an external hard disk that you plug into whichever computer you're working on.
But external disks are slow and fragile and cumbersome and take up extra space, a software solution would be better. Perhaps by now you're thinking "sure, use DropBox", and if you're a normal human being, that's perfectly good: DropBox is an amazing service, really well polished, I heard from lots of happy users. But if you're like me, DropBox seems expensive (like, you actually have to pay for any serious use), dangerous (my files on their server? never!) and not fun (can't hack on it). So, go opensource!
There's rsync, a gem of a program, excellent for mirroring a hierarchy of files. But if you actively change files in two locations and try to synch back and forth, it will, at the very least, recreate any file you remove. Not the right tool for the job then. There's also Unison, which seems to fit the bill, but it's big and weird and written in OCaml (what is that?!), I'd rather have something simpler. Some well-respected opensource backup tools (rdiff-backup, duplicity) might seem useful, but they are just that, backup tools, not good for keeping folders in synch.
So, then, I decided to build my own. There was a misguided attempt at a git-based FUSE filesystem, which was incredibly fun but hopelessly slow. But the second attempt, MagicFolder, I'm using quite happily to synchronize my music and photos and random files between home and work. It uses checksums to identify files, keeps a version history to figure out what to keep and what's safe to remove, and defers conflict resolution to the user by renaming one of the conflicting files. It needs to be run by hand, and weird things may happen if it gets interrupted during a synch, but with a bit of work those issues will be fixed.