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.
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.
TomP2P is a P2P-based high performance key-value pair storage library. Each peer has a table (either disk-based or memory-based) to store its values. A single value can be queried or updated with a secondary key. The underlying communication framework uses Java NIO to handle many concurrent connections.
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.
Forban is a P2P file sharing application for link-local and local area networks. Forban works independently from the Internet and uses only the local area capabilities to announce, discover, search, or share files. Forban relies on HTTP and is opportunistic, meaning that it replicates any files of interest seen in its proximity. The Forban protocols are minimalistic to ease the production of other implementations.
uhub is a high performance peer-to-peer hub for the ADC network. Its low memory footprint allows it to handle several thousand users on high-end servers, or a small private hub on embedded hardware. It uses the ADC protocol, and is compatible with DC++, jUCy, and other ADC clients.
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.