This month Patch Tuesday came and went quietly for Microsoft--they released no security related patches. Apple, however, released a huge update on Tuesday that fixes 45 vulnerabilities. If you have Mac OS X 10.4, these patches were released in the 10.4.9 update yesterday. If you are still running 10.3, the updates are part of Apple Security Update 2007-003. Because of the sheer number of vulnerabilities fixed, I suggest updating your computer as soon as you can. To install the update, run Software Update from the Apple menu--it will find all of the appropriate patches for your system. Earlier in the month apple released security updates for QuickTime and iTunes, so you may see a number of updates needed for your computer.
For more information about the Apple patches, I suggest the Apple Technical article or the CNET News.com article.