I've been exploring the world of radio emissions with rtlsdr since 2013 (http://superkuh.com/rtlsdr.html) and I'm still finding plenty of new things to do.

IMO, the newest cool stuff with rtlsdr are computational hacks to align multiple non-coherent receivers in time to do distributed direction finding and the like.

See also, https://ptrkrysik.github.io/

I cannot find the discussion, but the info is here: http://www.rtl-sdr.com/tag/rpitx/

Any links to write-ups/HOWTO's would be tremendously appreciated!

This is a pretty good write up of using the Pi as a wspr transmitter: https://github.com/JamesP6000/WsprryPi

Technically it works, but it's very noisy and the transmission range isn't far enough for anything too serious.

A full duplex SDR would be a better investment.

On the range, I did not pay much attention (but maybe I should have): the power of HackRF one is low, too; one needs an amplifier to transmit in the open air anyway. Do you think that the rpitx TX power is not suitable for table-top sized prototypes?

It says a dynamic range of only 13 dB on the levels.

https://github.com/theori-io/nrsc5

And a transmitter for GNU Radio.

https://github.com/argilo/gr-nrsc5

Still, this might be cool for subversive actions in countries with censored media, in an "Arab spring" like situation. Just a laptop, SDR and antenna on a driving truck and your on air (ofc I think there is not much overlap in places with widespread digital radio and places where you'd need to do this anyway)

https://www.youtube.com/watch?v=Ed7iy0gVywo

Still, I wish I had started with Novice so I'd have learned Morse early on.

I did it this way for the Tech/General/Extra, hammered all three of them out in the same session spending only $15 that Saturday morning.

Guess which one of us has the EE degree. :-) Kudos, that’s hardcore doing it all in one swipe. General took a little study for me, and after practically memorizing the questions, easily passed it. But Extra, damn, I’m going to have to bone up on some electronics before I pass that one. Or just do without the little sliver of frequency it gains me.

It is true that transmissions are regulated by law in every country, and the particular license you need will be determined by frequency band and power. In the US, there are some cases ("Part 15") where no license is required.

But even in something as common as WiFi you still have regulations, like channel 14, which as far as I know only works in Japan by default. Other common restriction is transmitting power.