Along the same lines as PropnetPSK, I ran across another system that looks promising, WSPR. Pronounced "Whisper", and is an acronym for "Weak Signal Propagation Reporter". This is the kind of thing that looks quite interesting, so I downloaded it and installed and ran it for a couple of days.
This looks to be a great mode. It decodes signals up to about -29 dB below the noise floor, uploads the spots to a realtime database and plots them on a google map. Most stations run about a watt or two, with some running a couple hundred milliwatts. This is an USB digital mode. Bandwidth is a whopping 6 hz! It does not take much power to be heard.
There are stations on most of the HF bands, however most are concentrated on 30 meters. Again, I would like to see some activity on bands like 2 meters, as being the extremely weak signal properties of this mode, it would be beneficial in showing enhancements on the VHF bands. So far, I have not seen any activity on 2 meters.
Although this is an excellent mode, I dont have a good working antenna for 30 meters, so my transmitted signals are typically down about 15 dB from others on the band. Until more activity starts to show up on bands above 10 meters, my activity on this mode will be limited, I believe. This is a personal preference, I would operate more if I had a 30 meter hamstick, which I may end up getting at some point, but for now, I will probably keep beaconing on PropnetPSK.