Go tournaments are generally run using a McMahon draw. There are several programs which can be used to control such a McMahon tournament. Some of these are listed in the BGA Software Catalogue [1].
The draw program recommended by the BGA for use in British tournaments is GoDraw V6 [2], written by Geoff Kaniuk. It runs under Windows.
It may be downloaded [2] and used freely. If you run a tournaments using GoDraw and make a profit, you are asked to consider making a donation to the Castledine-Barnes Trust [3].
If you run your tournament using GoDraw, there is a streamlined procedure for submitting your results to this web site and the EGF ratings [4].
Links:
[1] http://britgo.org/gopcres/gopcres1.html#s-draw
[2] http://britgo.org/downloads/godraw
[3] http://www.castledinebarnestrust.org.uk/
[4] http://britgo.org/tournaments/publicity.html#after