Shortly before the end of the year, Apple released Security Update 2007-009 to patch 41 vulnerabilities in Mac OS X 10.4.11 and 10.5.1. Many of these vulnerabilities can allow an attacker to take over your computer by simply sending you a malicious file or having you visit a malicious Web site. The affected applications and services include Address Book, CUPS (the printing service), and Quick Look.
(A few days after the initial Security Update 2007-009 was posted, Apple released version 1.1, which corrected a problem with Safari introduced in the prior update.)
Be aware that the Quick Look update will block any Quick Look plug-ins. We suspect this is a temporary fix and Apple will safely re-enable plug-ins in the (hopefully near) future.
A full listing of the updates can be found in. It's available through Software Update and in stand-alone form for  (a 35.6 MB download) and  (a 27.4 MB download) or  (a 15.9 MB download).
This is an extremely important update fixing multiple critical vulnerabilities. You should apply this as soon as possible, since many of these are the kinds of vulnerabilities favored by Internet attackers.