Security white paper for Leopard
Corsaire, who seem to be an IT security firm based in the UK, have released a white paper on improving the already good security of Leopard. It can be found on their page of technical white papers. Essential reading if you have Macs visible to the wild and woolly internet.
They wrote incredibly helpful white papers on securing Tiger and Panther, I found them to be essential when securing OS X Server for a public facing web and mail server. If you are still an OS version or two behind then these are also available at the link above.
I guess we might find it amusing that they have yet to release a white paper on securing either Windows XP or Vista but I am too polite to make the obvious jokes.