RetroShare is a Open Source cross-platform, Friend-2-Friend and secure decentralised communication platform.
It lets you to securely chat and share files with your friends and family, using a web-of-trust to authenticate peers and OpenSSL to encrypt all communication. RetroShare provides filesharing, chat, messages, forums and channels.

What is RetroShare?
RetroShare is the next generation sharing network, which provides:
* Reliable Identification and Authentication of your friends.
* Plus an Introduction Scheme which connects you to the friends of your friends, and facilitates network growth.
* Encrypted Communication, ensuring all shared information is known only to you and your peers.
* A Communication Platform which can potentially support services such as Secure Email, File Sharing, Streaming, Video or Voice over IP, Photos, Wall and Messaging
* A Decentralised Social Sharing Network designed **FOR THE PEOPLE** with no dependancies on any corporate system or central servers.
* Retroshare's philosophy: decentralization and privacy.

Why use RetroShare:
* You want to chat and share files securely with your friends
* Only your friends will be able to see and download files that you share.
* RetroShare is serverless, which means - unlike other messengers, you don't* need to register, complete annoying registration forms and receive tons of ads. Sending a key to your friend by e-mail is enough to set up your own IM network. This also means no IM spam, because people you didn't invite absolutely cannot connect.
* You can also use your favorite nick name either: no need to use cryptic names like "john.smith148@something.com" just because someone (whom you don't even know) snapped your favorite nick before you.
* RetroShare is encrypted, meaning high privacy: nobody, including your
ISP, can see what files you're sharing.
* You need secure instant messaging and files exchange for a small workgroup.
* You need to chat securely and exchange files with other colleagues at work, but do not trust your data to a public chat network like Live or Google Chat ?
* Don't want a hassle of installing and maintaining your own chat server either ? Use RetroShare instead:
- RetroShare is serverless , which means that no server, ever, gets to see your data. Any IM server, no matter how secure, poses a risk of data leak because of hacker's attack. Retroshare completely eliminates this security risk associated with the IM server, without any added costs.
- RetroShare encrypts all connections, which means every bit of data is encrypted end-to-end. Unlike other products using self-made weak encryption, RetroShare uses a special version of industry standard Openssl library, trusted by banks and other organizations.
- Absolutely no vendor lock-in: you own your RetroShare network. There is no 3rd party vendor whom you have to pay, or who may go out of business and force you to migrate to another product.

Features
* Serverless, completely decentralised
* Multiple simultaneous downloads / uploads
* Search Friends
* Messages
* Forums
* Channels
* Voice over IP
* Instant messaging
* Groupchat
* GnuPG Authentication
* OpenSSL Encryption
* adding downloads via website links
* Plugins support
* UPnP / NAT-PMP port forwarding support
* Graphical User Interface written with Qt4 toolkit
* System tray integration

Anonymity and Censorship Resistance beyond your own friends
Retroshare's security comes from using a set of communication algorithms that provide anonymity beyond your own friends: file transfer is ruled by a Friend-2-Friend routing algorithm, decentralized chat is made possible by an anonymous message routing method.

The benefits of such algorithms are best described by the author's of Turtle, one of the algorithms Retroshare uses:

When designing Turtle, we were inspired by the way people living under oppressive regimes share information deemed "hostile" by their government (this can be books, newsletters, video and audio recordings, or even political jokes). Because of the potentially very serious consequences raising from being caught possessing/distributing such material, no single individual is willing to share it, except with close friends. Experience has repeatedly shown that, even in the most repressive environments, this "friends-to-friends" delivery network is remarkably effective in disseminating information, with relatively little risks for the participating parties; if one chooses his friends carefully, the chance of being caught doing the forbidden exchanges becomes very small.

Retroshare's decentralised model means there is no central authority to remove content. As described by Wikipedia, Retroshare "... makes it difficult to intrude and hardly possible to monitor from an external point of view." This makes government monitoring and prosecution impossible beyond the will of Retroshare users.