PSK31 by G3PLX
The following link is an article titled "PSK31: A New Radio-Teletype Mode" By Peter Martinez, G3PLX. The article appeared in July/Aug 1999 edition of QEX:
http://www.arrl.org/files/file/Technology/tis/info/pdf/x9907003.pdf
Digipan Software
PSK31 The Easy Way by Alan J Gibbs, VK6PG/G3PHG