I love PSK31. It's my preferred mode of contact over all others, voice or digital. It suits my low power station and modest antenna configuration perfectly, and I can decode a signal that's barely visible on the waterfall. My most prized PSK31 QSO to date has been on 20 meters with ZL3KR in Christchurch, New Zealand (about 9,000 miles).
Along the way I've met some interesting characters, like the Old Testament professor at a German university, a large animal vet in Colorado, and the guy in Alabama who slammed into another car during a head-on collision, leaving an imprint of his son's face on the dashboard of his truck (he's OK).
PSK31 (Phase Shift Keying with a bit rate of 31) was created by Peter Martinez (G3PLX) in 1998. Using little more than a computer sound card, a SSB radio and a few cables, PSK31's strength lies in its ability to overcome poor propagation. It accomplishes this by transmitting a series of zeroes at the beginning of each transmission so both transmitter and receiver are synchronized. Moreover, the structure of the varicode data itself is mathematically predictable so even if the transmission is dropped mid-QSO the receiving station can easily resynchronize and continue decoding the message. In spite of its exceptional weak signal performance, difficulties sometimes occur over transpolar paths where the signal phase can be adversely affected by auroral events.
PSK31 doesn't use error correction and it's relatively slow, so it isn't suitable for mission critical files, like sending executable applications. On the other hand, it hums along at a reasonably comfortable typing speed of about 50 words per minute so a QSO can sustain a lively conversation.
My hardware setup consists of an ICOM 706MKIIG, a Dell laptop and a Rigblaster Pro interface. While homebrew equipment is fine, the Rigblaster greatly simplifies the interface between the computer and transceiver. I generally use about 50 watts, an output that I've successfully used many times during routine 4,000 mile QSOs with Europe.
I use Digital Master 780, part of Ham Radio Deluxe, for encoding and radio control. The SuperPSK function lets me easily see who's on the band, and I can shoot my completed QSOs right into my log book.
My preferred operating band is 20 meters, right around 14.070. I've recently begun a foray into PSK contesting which allows me to rack up as many as 100 contacts per hour. Nonetheless, I still prefer a more leisurely QSO where I can get to learn something about the other guy. After all, isn't that what ham radio is supposed to be about?