Critter usage is free of charge [1-2]. Critter source code is available upon request [3].

[1] http://www.vlasak.biz/critter/
[2] http://www.schach-welt.de/schach/computerschach/interviews/richard-vida
[3] http://www.talkchess.com/forum/viewtopic.php?topic_view=threads&p=412971&t=39577