I have had many hours of fun over the past few years, transmitting very weak WSPR signals at <0 dBm (1 milliwatt) output power.  My most recent "experiments" were at a level close to -20 dBm, aka 10 microwatts.  Still spotted a few hundred miles away by a few stations.  I do wish there were a way to advertise power levels lower than 0 dBm, the lower limit in the current WSPR protocol.  I assume the protocol can't easily modified to do that without breaking the decoding ability.