Psi is a GPLed instant messaging client for the Jabber (XMPP) protocol which uses the Qt toolkit. It runs on Linux, Microsoft Windows, and Mac OS X. Ready-to-install RPM and deb packages are available for many Linux distributions. Successful ports of Psi were reported for FreeBSD and Sun Solaris operating systems. Due to Psi open-source nature, several private (unofficial) builds have appeared, which usually contain features that will appear in future official Psi versions.
Psi (pronounced "sigh" [saɪ]) is an abbreviation of the word "psynergy", which is like psychic energy. Since 'psi' is also the twenty-third letter of the Greek alphabet (Ψ), that's why Psi uses the Greek symbol as the logo.
The application was created by Justin Karneges and it began as a side project. At various points during its existence Justin was paid to develop the codebase, during which Psi flourished. Typically however, the release cycle of Psi is relatively slow, but the client has always been seen by its fans as a very stable and powerful instant messaging client. Justin left the project (http://psi.affinix.com/forums/index.php?act=ST&f=5&t=2278) in late 2004 to pursue other endeavors, but he is still well-known as the original author.
Because Jabber allows gateways to other services, which many servers support, it can also connect to the Yahoo! Messenger, AIM, ICQ and .NET Messenger Service networks. Other services available using gateway servers, include RSS and ATOM news feeds, sending SMS messages to cellular networks and weather reports.
As of 2004, Psi has language packs for 14 languages, with even more being created.
Emoticon packs are supported using the jisp format. Many jisp emoticon packs are available, including ones from AIM, iChat, and Trillian.
Since version 0.9.2, Psi supports file transfers between other Jabber clients. However it's not possible to send to or receive files from other IM networks. Version 0.9.3, released in January 2005, supports Contact Is Typing Notification (which works even with MSN and AIM contacts) and fixes many usability issues of previous versions.