Apple has updated Mac OS X for both Intel- and PowerPC-based Macs, bumping its operating system up to version 10.4.5. The patch incorporates a number of fixes and improvements for the OS itself, along with a number of compatibility tweaks for third-party and Apple-branded applications. It also includes Apple's most recent security updates.
More information about the update's particular changes can be found here. There's a version of the installer for systems already running Mac OS X 10.4.4, which weighs in at 16MB for the PowerPC release and 98MB for the Intel version - get them here and here, respectively.