filegive easily sends or receives files point-to-point, with authentication and ciphering, and the other side only needs a Web browser. No third party server is involved in the transfer. It can use common NAT traversal protocols like uPnP and NAT-PMP, manually forwarded ports, or a public ssh server.
robonobo is social music sharing and downloading software. It allows you to share your friends' music libraries and playlists, stream the audio instantly, and download the file in the background so you can burn it, export it to your music player, or do whatever you want with it. You can import your friends list and share links to your playlists via Facebook and Twitter.
Proximate is a social peer-to-peer (P2P) application designed for local device to device networking. It provides social communities with several user functions: Messaging, file sharing, and message boards. The application is designed to be completely P2P, i.e. servers are not needed. It only assumes TCP/IP networking, which means that it works over wireless/wired links such as ad hoc WLAN and Ethernet LAN. The application is written in Python for GNU/Linux systems with a GTK+ GUI, Nokia N900 being the primary platform.
KTorrent is a BitTorrent application that allows you to download and share files using the BitTorrent protocol. Key features include queuing of torrents, global and per-torrent speed limits, previewing of certain file types, importing of partially or fully downloaded files, file prioritization for multi-file torrents, selective downloading for multi-file torrents, kick/ban peers with an additional IP Filter dialog for list/edit purposes, UDP tracker support, support for private trackers and torrents, support for µTorrent's peer exchange, support for protocol encryption (compatible with Azureus), support for creating tracker-less torrents, support for distributed hash tables (DHT), support for UPnP to automatically forward ports on a LAN with dynamic assigned hosts, support for webseeds, scripting support via Kross, and interprocess control via DBus interface.
RHash is a utility for computing hash sums and creating magnet links. It supports CRC32, MD4, MD5, SHA1/SHA2, Tiger, TTH, Torrent BTIH, AICH, ED2K, GOST R 34.11-94, RIPEMD-160, HAS-160, EDON-R 256/512, WHIRLPOOL, and SNEFRU hash algorithms. Hash sums are used to ensure and verify the integrity of large volumes of data for long-term storage or transferral. RHash can calculate magnet links and EDonkey 2000 links. It can update existing hash files by adding sums of files missing in the hash file. It can output in a predefined (SFV, BSD-like) or user defined format. It can process directories recursively.
Bitcoin is a peer-to-peer electronic cash system that is completely decentralized, without the need for a central server or trusted parties. Users hold the crypto keys to their own money and transact directly with each other, with the help of a P2P network to check for double-spending.