Official PSK31 Site
A digital mode useful for keyboard to keyboard QSOs. Works at 31.25 bauds. Easy to use and monitor. Gives good copy under poor SNR and is thus suitable for QRP. Uses BPSK or QPSK.
Resource for PSK31 setup and operation. Email discussion list.
ARRL: PSK31 Spec
Provides information on PSK31 including varicode characters, convolutional code and Viterbi decoding.