PSK-31 is a recent digital mode that is very efficient in terms of bandwidth use and is excellent for keyboard to keyboard chatting on noisy HF.
I've been listening around for the distinctive warble sound on 80m and 40m for some time and hadn't found anything I could decode, then I read about 14.070Mhz (20m) and that seems to be where the action is. Several strong stations were heard without trouble.
A simple wire dipole for 20m was constructed and strung between two trees. I have a Yaesu FT-817 portable rig which has a mini-din 6 data jack on the back so an interface box (pictured above) was constructed. As I run a Mac the excellent cocoaModem software is in use. To key the transmitter, (as the FT-817 doesn't support VOX on the data port - a great pity) I first tried making a simple vox circuit but in the end used cocoaPTT which toggles the RTS line on a USB serial cable. One diode was used to save the radio from the +ve swing, so it just pulls PTT low to transmit, otherwise no electronics, just soldering, seems to work fine.
I called CQ, running just 5W into a very flaky antenna and was immediately called back by JA2LCN in Ogaki City, (this person seems very active).
If you run Windows there is lots of software about for PSK31, there's also some Linux software with a reasonable GUI.
In use, you leave the radio tuned to 14.070Mhz and watch the waterfall display. When you see a signal you click between the lines and start reading. It seems like a very nice way to chat, particularly if you can touch type. (A lot of receive errors I noticed now seem to be bad typing in retrospect).
I note there's some interesting kits around for minimal, low power transceivers designed for PSK31 use specifically. It's interesting to consider that a little board like this, a roll of wire, antenna tuner and a laptop and you can chat half way around the world pretty easily.