Quaternion is a cross-platform Qt5-based desktop IM client for Matrix, a reference desktop application using libqmatrixclient SDK. Source code, issue tracker, roadmap etc. can be found at GitHub. The latest release (with links to cross-platform source code archives, as well as archived binaries for Windows) can be found at the GitHub Releases page.
Most of talking around Quaternion happens in our Matrix room: #quaternion:matrix.org. Welcome, and have fun!