Write a note! Good for networking with other people in the guestbook after you leave.
Great if you want to find other like minded hackers around the world creating hackerspaces or other types of projects inspired by the Noisebridge principles.
(Nov 2016) - Gavin from https://twitter.com/yorkhackspace.
(Nov 5th 2016) - Lambert from Helios Makerspace, http://heliosmakerspace.ca/
(Nov 16th '16) Yoni from Berlin https://twitter.com/shushugah