Andrew Birkett's nobugs.org
This is gocaml, a graphical frontend for GMP (Go Modem Protocol) engines. Although usable, it has a very minimal user interface and is intended more as an example of ocaml programming than a serious Go client. NOTE: It does not provide a computer player itself – it needs something like gnugo for that.
It was written to explore the ocaml language, and also because I couldn’t get cgoban to run on my linux IPAQ. It was almost completely written natively on the IPAQ (with folding keyboard) using emacs and ocamlc. Great performance, great language.
Also have a look at my oclisp-0.5 (lisp interpreter). You can find lots more cool ocaml software at the ocaml links database and the caml humps.
You can download the source (18k) for gocaml-1.0 here.
Gocaml running on a windows box:

And here’s a snippet from the GMP log:
Note: 'lsn' is local sequence number and 'rsn' is remote sequence number. This avoids the role-reversal headaches of the GMP hsn and ysn. Gmp: RECV [lsn=0,rsn=1] New game Gmp: SENT [lsn=0,rsn=1] Ok Gmp: RECV [lsn=0,rsn=0] Query for rule set Gmp: SENT [lsn=1,rsn=0] Answer of 1 Gmp: RECV [lsn=1,rsn=1] Query for board size Gmp: SENT [lsn=0,rsn=1] Answer of 19 Gmp: RECV [lsn=0,rsn=0] Query for handicap Gmp: SENT [lsn=1,rsn=0] Answer of 9 Gmp: RECV [lsn=1,rsn=1] Query for computer player color Gmp: SENT [lsn=0,rsn=1] Answer of 2 Gmp: RECV [lsn=0,rsn=1] Ok Gmp: RECV [lsn=0,rsn=0] White move at 118 Gmp: SENT [lsn=0,rsn=0] Ok Gmp: SENT [lsn=1,rsn=0] Black move at 119 Gmp: RECV [lsn=1,rsn=0] Ok Gmp: RECV [lsn=1,rsn=1] White move at 63 Gmp: SENT [lsn=1,rsn=1] Ok
Gocaml is released under the GPL.