An Avalonia MSNP15 client. It's compatible with Linux, Windows and MacOS.
The default server is CrossTalk, which is currently in private alpha. That can be changed in the options window, however, to any server supporting version 15 of the protocol.
For the notifications feature to work on macOS the client has to be signed. As the $99 yearly developer ID price is too much for me (especially in my currency), I used a self-signed certificate. So in order to get notifications working you need to install and trust this certificate, which will come with the app bundle as a .cer file when downloaded.