postmodem

Agreed, the UI is very clean and polished. (I use the Android version.)

I doubt that it'll going to happen soon: https://threema.ch/en/faq/source_code

On the other hand, the web client they're about to release _is_ open source.

Anyway, it's the most secure messenger at the moment, IMHO, and I love the fact that no phone number is required.