mlgo is an client written in objective caml. Although in early developement it can be used to play gnugo or some other gtp-engine. There is more info at the homepage.
vbmithr: The link is dead. Anybody has downloaded this program or knows the author ? Thanks.
JimB: The comment above was added in 2008; the newest link I can find is http://repo.or.cz/w/mlgo.git/shortlog, dated 2008-10-18. I think this project is dead.