bittorrent-sync

 Bittorrent Sync  is a proprietary Dropbox replacement/alternative. It is different than most dropbox/box services in that it is distributed and central servers are optional, but are used to speed up peer discovery. Files are synchronized between devices directly or through a public relay server. Shared folders are connected using secret keys. There are a read only keys and read/write keys as well as one time only keys of both flavors for transmitting to someone over an insecure connection.

Because there are no central servers, uncommon connection strategies are required for reliable peer connections:


 * A bootstrapping server for finding peers (known as a tracker).
 * A relay server for peers that cannot make a direct connection.
 * Local peer discovery over the LAN
 * Peer Sharing.
 * DHT (distributed hash table) peer discovery for trackerless peer discovery.
 * NAT-PMP/UPnP For automatic port forwarding.
 * A trust system based on secret keys instead of passwords and accounts.
 * Uses bittorrent as the transfer mechanism so the more peers/devices you have, the faster speeds you can achieve.

All strategies come preconfigured and can be disabled individually or customized to use custom tracker or relay servers.

http://www.bittorrent.com/sync

torrent
A torrent file is a file used by BitTorrent, a peer-to-peer (P2P) file sharing program, for downloading a file (often from multiple locations at one time).