There is a new very weak signal beaconing mode being trialled currently on 136 and 500kHz called OPERA. The current version is available as a zip download at http://www.mediafire.com/?w74a061m48nv806 but this is a beta version and updates are happening quite fast at the moment, so best check the RSGB's LF reflector for the most recent version from Graham G4WGT.
Initial indications are this is even better than WSPR, which as we all know is an excellent QRP beaconing mode. I managed to decode SM6BHZ (just under 1000km) and PA0A tonight when using just my 20m separated earth electrodes (no antenna in the air here at all) on 136kHz.
Over the weekend I may give this a go with a few mW ERP on 500kHz and, when I get my vertical loop re-erected, on 136kHz too.
Early days yet, but a most interesting mode that does NOT need critical PC timing (unlike WSPR) and that can use the PTT line or VOX control to key the TX i.e. it is simple to interface rig to PC.