Extremely low-power FM transmitters in the licensed FM broadcast band are legal in the EU since a few years.
Umm, that made me curious - do you have any pointers?
I was under the impression that anything broadcasting in these broadcast bands required a specific license, period. The power output was irrelevant, there was no exception for these low power devices. I am quite sure this is still the case in Slovakia where I am from. On the other hand, Slovak rules tend to be incredibly backwards compared to the rest of the civilized world ...
Correction - I think I have found a recent change: It seems that the law was indeed changed in 2012 (implementing an EU resolution 2010/368/EU from 30.6. 2010) and if the ERP < 50nW, license is not required in the FM band. However, there are two gotchas - first, wireless microphones are explicitly banned and second, this sort of "bug" can certainly exceed 50nW without special attention ... Also there are some explicit requirements about what the device needs to satisfy, I am pretty sure that rules out any "homebrew" stuff without the right paperwork.
But you can still get into trouble for violating other regulations, like lack of certification, or interfering with aircraft communication. Or, an extremely nasty one, if there is a local law against bugs, listening devices, surveillance devices etc.
I didn't want to get into this, but yes, it is obviously illegal to bug someone or cause interference due to poorly tuned/shielded oscillator (air bands are right next to the FM broadcast band in many places!).
To conclude, if you want to play with radio, the best way is to get the basic HAM radio license and then you can play legally in amateur bands. All that takes is passing a simple test and paying the fee for getting the paperwork issued.
J.