Syphon
Syphon is an open source Matrix client which currently supports E2ee (End to end encryption), Sso (Single sign on), Multi Account and Multi Language. Currently, Syphon is in alpha, with the latest version being 0.2.0, with 0.3.0 being in progress. This version will bring cross signing, Key Importing and Exporting, a stable desktop build, timelines having state events and message formatting.
Their source code can be found on GitHub page.