Although Apple currently focuses its efforts - in a totally logical way - on Lion and Mountain Lion, the truth is that they do not forget the past operating systems when it comes to security.
Specifically Leopard has been the operating system that has been updated with security patches to prevent the team from malicious software and especially the dreaded Flashback, a Trojan that has affected a large number of Apple computers.
Here are the two patches released for Leopard:
Source | Macrumors
They are expensive, I have an old equipment that only supports snow leopard, I live in Cuba, so it is more difficult to acquire a more modern one, that's why I take care of it, and I recommend it for what it is, one of the best options to connect life with the world