Attention!

1) All last builds of Psi for Windows and macOS were done without PsiMedia plugin (for audio and video calls). Support of this plugin for Windows and macOS will be re-added in Psi 2.0.
2) OMEMO plugin will be available in Psi only starting from 2.0 release. If you need this plugin right now, use official builds of Psi+ project (development branch of Psi). See details in README.
3) Psi 2.x releases will be the last releases with support of Windows XP and Ubuntu 16.04.

Official Psi 1.4 Downloads

Microsoft Windows (64-bit portable version)
70.2 MB portable version
Requires Windows Vista or newer
Microsoft Windows (32-bit portable version)
68.2 MB portable version
Requires Windows Vista or newer
macOS (basic version)
80.7 MB disk image
Requires macOS 10.9 or newer
macOS (webkit version)
80.7 MB disk image
Requires macOS 10.9 or newer

Official Psi 1.3 Downloads

Microsoft Windows (64-bit installer)
84 MB installer
Requires Windows Vista or newer

Other downloads including builds for Windows XP, experimental and archived builds for Windows, macOS and Linux are available on SourceForge.

Source Code

Sources
2.1 MB archive
Requires Qt 4.8 or Qt 5.5+
Translations
2.9 MB archive
Requires Qt 4.8 or Qt 5.5+
Plugins
822 KB archive
Requires Qt 4.8 or Qt 5.5+
Psimedia
210 KB archive
Requires Qt 4.8 and GStreamer 0.10.x

Sources for old and for future releases are available on GitHub.