Utility for syncing a folder across two devices?

Question

I basically want to make my own Dropbox, but with my own drive (probably on a Raspberry Pi) as the server. Is there an existing daemon that will just keep a pair of folders on different computers in sync? I know there are things like SMB, but that doesn't leave you a local copy when you go offline.

WaltPurvis · Accepted Answer

Syncthing: https://syncthing.net/

shoo · Answer

Unison: https://www.cis.upenn.edu/~bcpierce/unison/

mceachen · Answer

Resilio Sync: https://www.resilio.com/individuals/

juangacovas · Answer

lsyncd: https://axkibe.github.io/lsyncd/

gshdg · Answer

rsync