Note that Fairy-Max, the default demo engine that comes with WinBoard, now also offers Seirawan Chess (since WinBoard 4.5.0). Other variantssupported by Fairy-Max are
Makruk (Thai Chess)
Shatranj (Ancient Arabic Chess)
Courier Chess (Medieval precursor of the current FIDE game)
Berolina Chess
Cylinder Chess
Gothic Chess
Capablanca Chess
Knightmate
Spartan Chess
Chess with Different Armies (FIDE, Clobberers and Nutters armies selectable)
If somebody is interested in an S-chess program, I have released my Zillions S-chess program here:
http://hem.passagen.se/melki9/seirawanchess.htm
/Mats