Osmo-FL2K can be considered as the [evil] transmit-side brother of RTL-SDRs. It is a driver that allows cheap $5 - $15 USB 3.0 VGA adapters to be used as a transmit-only capable SDR. It might be considered [evil] as transmitting illegally and without filtering can pollute the RF spectrum, but being responsible with it and using appropriate filters could enable extremely low cost transmitters.
Recently at the October 2018 Osmocom Conference, Steve M, the man behind the Osmo-FL2K discovery and software (and heavily responsible for the development of RTL-SDR too) has given a talk titled "osmo-fl2k - the [evil] transmit-side brother of RTL-SDR". In the past he's also given a similar talk that we posted about previously.
The talk goes over the discovery and reverse engineering of Osmo-FL2k, discussion of the application itself, some signals that have been successfully transmitted and some measurements.